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Bolu Kukus (Indonesian Steamed Sponge Cake) has the texture of sponge cake but it is made by steaming it rather than baking it. You can find it anywhere in Indonesian local markets. It is cheap and easy to find and thus, my mom never made it. I had to learn it from the internet. It took me 3 attempts to get good ones I liked. On my first attempt, I didn't use a new unopened bottle of sparkling water. I also had to improvise to make the mold. Traditionally, it is made using Aluminum bolu kukus mold because Al can conduct the heat better. The mold has holes on the bottom and side so that the steam can cook the batter. However, I didn't own one and I couldn't find it outside Indonesia. So, I used Aluminum cupcake liner and punched some holes on the bottom and the side. The resulting cakes didn't rise as tall as it was supposed to because the cupcake liner was not sturdy enough to provide structure for the cakes to rise. On the second attempt, I used a new unopened bottle of sparkling water and 2 different mold types (individual Aluminum muffin/cupcake tin and individual silicone muffin baking cup). For the Al muffin tin, I punched the holes while for the silicone muffin baking cup, I used scissors to make the holes. Please see the picture above (red cups) to see how I made the holes on the silicone baking cup. The resulting cake on my second attempt was better in term of rising with the silicone cup having a better result. This is because the silicone cup could maintain the structure better in the heat. The Al tin was slightly expanded in the heat. I noticed all cakes were dense. As mentioned above, bolu kukus is supposed to be light like sponge cake. I figured out it was probably because I mixed the batter too much. I also learned on the internet that during steaming, we are supposed to put kitchen towel around the lid to absorb the moisture so that the water from the moisture doesn't drop on the batter affecting it to rise. On my third attempt, I only used silicone baking cup and a new unopened bottle of sparkling water, covered the lid with the kitchen towel, and folded the flour rather than using a mixer. I also gave enough space between each mold so that each cake had enough space to rise and open. The resulting cakes were not dense and rose wide and open. So, all my efforts paid off. Based on what I learned, I recommend you to use silicone muffin baking cup (if you don't have the traditional Aluminum bolu kukus mold), a new unopened bottle of sparkling water, and kitchen towel to absorb the moisture and fold the flour. Before using the silicone muffin baking cups, use scissors to make holes on the bottom and side of the cups. You also need to give enough space between each mold so that each cake can rise and open without space restriction.

When I made the cakes, I used green, yellow, and brown colors. However, you can use any colors you want. Red will look nice. Unfortunately, I didn't have it when making them. If you have never seen how bolu kukus is made, you won't know how exciting and fun it is. This is because when it is being steamed in the steamer, the cake rises and blooms open like flower. I always thought bolu kukus is visually pretty like flower. However, it is literally blooming like flower when it is being steamed. My steamer has a glass lid, so I can watch it blooming while cooking. In addition, the color pattern you put on the top surface of the mold determines how it will look like at the end. So, try different patterns from 1 cake to another and you will get different end results. Putting different color patterns, Watching colors bloom open during cooking, and seeing the end result of the cakes make this a good project to do with your children. They will enjoy it and have fun.

The recipe makes about 9 bolu kukus.

Ingredients:

• 2 egg whites at room temperature
• 3 egg yolks at room temperature
• 250 g sugar
• 1 teaspoon vanilla extract
• 1/2 teaspoon baking powder
• 275 g of all purpose flour
• 225 ml sparkling water. Use a new unopened bottle of sparkling water since it affects how the cake rises.
• 2 drops yellow food coloring. You can also use any other color.
• 2 drops green food coloring. You can also use any other color.
• 1/4 teaspoon cocoa powder
• Tool needed: stand mixer. You can also use hand mixer.
• Tool needed: rubber spatula
• Tool needed: steamer
• Tool needed: 4 individual silicone muffin baking cups. Using scissors, cut several holes at the bottom and side of each cup. Make sure the holes are evenly distributed on each cup so that it can cook evenly.
• Tool needed: 9 cupcake liners

Methods:

1. Using stand mixer, add egg whites and egg yolks to the mixing bowl. Start the mixer and set it to the highest speed. Run it for about 2 min. While the mixer is running, add sugar little by little and vanilla. After adding the sugar and vanilla, let the mixer run at the highest speed for 20 min. If you don't have stand mixer, you can use hand mixer but you need to use the highest speed when mixing.
2. While the mixer is running, you can use the time to sift the flour and baking powder.
3. After 20 min, turn off the mixer.
4. Add 1/3 of sparkling water and 1/3 of flour mixture into the egg mixture in the mixing bowl. Using rubber spatula, gently fold the flour mixture into the egg mixture batter so that the flour is incorporated in the batter. Once the flour is incorporated, repeat this step until all sparkling water and flour mixture have been used.
5. Take about 10 tablespoons of the batter and put it in a small bowl. Add yellow food coloring. Using clean rubber spatula, fold the batter so that the color is incorporated. Take about 10 tablespoons of the batter and put it in another small bowl. Add green food coloring. Using a clean rubber spatula, fold the batter so that the color is incorporated. Take about 10 tablespoons of the batter and put it in another small bowl. Add cocoa powder. Using a clean rubber spatula, fold the batter so that the color is incorporated.
6. Fill up your bottom steamer with water. Put the bottom steamer on the stove and turn on the stove to high. Put the lid on the steamer.
7. While waiting for the water in the steamer to boil, you can use the time to put the batter in the silicone baking cup. Put cupcake liner inside each silicone baking cup. Make sure the liner is placed snugly inside the cup so that it follows the shape of the cup. Since silicone baking cup is soft, we don't want to manually handle and move it after filling it up with the batter. So, place the top steamer on kitchen counter, put silicone cups at the top steamer, and we will move the top steamer to the bottom steamer once we are done filling up the cups. Make sure you provide enough space in between each cup since the batter needs some spaces to rise to the side. So, give at least 2 " in between each cup. If your steamer is small, just put as many cups you can put that meets the 2" space requirement. Mine can fit 4 cups. For each cup, fill half of the cup with the batter that has no food color. Fill up the rest with the other colors. I normally put the left side with 1 color, the center with another color, and the right side with another color. Make sure you fill up the cup all the way to the top so that it can rise nicely. You can try many different color patterns and see how it looks like. It is fun to try different patterns and get different results. Once the water in the bottom steamer is boiling, remove the lid from the bottom steamer and place the top steamer on top of the bottom steamer. Place the lid on the top steamer. Place kitchen towel around the edge of the lid to absorb the moisture. This way, the water from the moisture will not drop to the batter during cooking affecting how it rises. Make sure the kitchen towel doesn't touch the fire or otherwise, it would catch fire. Let the cakes steam for 15 min. During cooking, do not open the lid at all since it will affect how it rises. After 15 min, remove the top steamer. Remove the lid from top steamer. If you still have some batter left, check if you need to replenish the bottom steamer with water. If so, replenish it with water and put the lid on the bottom steamer. Using kitchen tongs, remove the silicon cups from the top steamer and set them on the cooling rack. Let the cakes cool down for 1-2 min before you remove it from the cups. If you still have batter left, repeat this step.
8. Once all the batter has been used up. Turn off the stove.
9. Serve bolu kukus. If you are not serving them right away, let them to completely cool first . You can then keep them in a tight container for 1 day. Don't leave them in open air since they will dry up and become hard.

Notes:

• Make sure you use unopened bottle of sparkling water since it affects how it rises.
• Always sift the flour and baking powder before adding them to the egg mixture.
• To get good rise, use a mold that is sturdy. I found individual silicone muffin baking cup better than individual Aluminum muffin tin because it holds the shape better. Make sure you make some holes at the bottom and side of the cup so that the steam can cook the batter evenly. Silicone cup is soft and thus, it is harder to move around without spilling the batter after you fill it up. So, I place the top steamer on the kitchen counter, put the empty silicone cup with the cupcake liner on the top steamer, and fill up the batter while it is inside the top steamer. Once I am done, I just move the top steamer to the bottom steamer. That way, I don't touch the cup at all once it has been filled with the batter.
• Do not use mixer to mix the flour mixture with egg mixture. If you over process them in the mixture, the cake will be dense. Fold the flour mixture in the egg mixture using rubber spatula instead.
• Place kitchen towel around the edge of the steamer lid while steaming the cakes. This is to catch the moisture so that the water from the moisture doesn't drop on the batter affecting how it rises.
• Do not open the lid during cooking. Otherwise, it could affect the rise of the cake. It takes about 15 min to cook the cake. So, just open the lid after 15 min.

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