My first (unsuccessful) attempt used this recipe I found on a blog - I followed the instructions to the letter (used the right kind of baking powder, didn't overbeat the eggs, etc.) - but I ended up with a deflated, mushy, compact gelatinous brick. It also wasn't sweet enough - blech! - the results went into the trash.
For attempt #2, I thought, "Maybe the cake doesn't have enough gluten to provide structural support?" So I substitute cake flour for half of the tapioca flour, increased the sugar, and ended up with something that tasted more like a Pandan pound cake - good, but no honeycombs - and it definitely wasn't Banh Bo Nuong.
Attempt #3 utilized a combination of tapioca flour, rice flour, and a small amount of cake flour - I was hoping the rice flour would give a chewier texture and also help with the structure of the cake to prevent it from collapsing. Nope, wrong again! Cake #3 was good - but still was nothing more than a Pandan pound cake, and still did not resemble Banh Bo Nuong.
Attempt #4 finally gave me results worth posting. I decreased the amount of coconut cream, increased the sugar, went back to all tapioca flour... plus I increased the oven heat, and implemented chiffon cake strategy when cooling the cake. Success! Here is what I learned:
There are several steps that must be strictly adhered to when making this cake: First, use only SINGLE-acting baking powder. Most American markets stock only DOUBLE-acting baking powder - which is great for banana bread - but will ruin your honeycomb cake. You can find little pink packets of Alsa brand (single-acting) baking powder at Asian markets. Or, make your own baking powder by combining cream of tartar and baking soda in a 2:1 ratio. My successful cakes #4 and #5 both used cream of tartar + baking soda, so I know that it works.
Second, make sure you preheat your cake pan, greasing only the bottom. Leave the cake pan sides un-greased, to give the cake something to cling to for support. Angel food cake pans are great for this cake, as the center tube will give added support. If your cake lacks the honeycombs, it could be that your oven is not hot enough, or could be out of calibration - experiment with baking at 10 degree hotter temperatures until it works. Once baked, hang the cake upside-down on a rack to cool, if possible. This will help reduce the likelihood of your cake collapsing when cooling.
Third, be careful not to beat any air into your batter. The trapped air will expand in the oven, causing the cake to rise... and the more air that is incorporated, the more the cake will deflate as it cools. Stir your ingredients slowly - resist the urge to use a beating or whipping motions.
Tapioca starch and tapioca flour are the same thing - use either. You can find tapioca flour at an Asian market, or at a "Whole Foods" type of health food market. I bought my latest batch of tapioca starch at Sprouts Market, Bob's Red Mill brand. Coconut cream is thick coconut milk. You can sometimes find coconut cream for sale in cans, like I did. Or, buy coconut milk, and do not shake or agitate - open the can, and use the thick layer of cream that has floated to the top of the can.
1/2 c. coconut cream
1 1/4 c. sugar
1 1/4 c. tapioca starch
2 1/2 tsp. single acting baking powder (make your own with 1 3/4 tsp. cream of tartar + 3/4 tsp. baking soda)
1/2 tsp. salt
1/2 - 1 tsp pandan extract
1 tsp. vanilla extract
1. Preheat a convection oven to 360 degrees (375 degrees in a regular oven). Grease the bottom (not the sides) of an angel food cake pan with canola oil, and allow the pan to preheat inside the oven. (You want the pan to be nice and hot when you add the cake batter.)
2. Sift the tapioca starch, and baking powder. Do not use double acting baking powder, or this recipe will fail.
3. In a separate container, stir the eggs until homogenized - you want to avoid beating any air into the eggs.
4. Mix the coconut cream, sugar, pandan, and vanilla extracts. Stir until sugar is dissolved.
5. Stir the eggs into the coconut milk syrup. Do not beat or overly agitate the mixture.
6. Pour the wet ingredients on top of the dry ingredients, and mix to combine. Stop mixing when you still have some lumps - over mixing will ruin this cake.
7. Pour the batter through a sieve, and using the back of a rubber or silicone spatula, push all of the batter through. This step will take care of all the lumps, allowing a nice smooth batter, without over mixing. Sieve the batter 2 more times to ensure a perfectly smooth batter.
8. Pour the batter into the hot cake pan, and bake for 10 minutes at 360 degrees in a convection oven (375 degrees in a normal oven). After 10 minutes, reduce the heat to 305 degrees, and continue baking for 25-30 more minutes. Test with a wooden toothpick - when the toothpick comes out clean, the cake is ready.
9. Remove the cake from the oven, and invert onto a wire rack. Allow to hang upside down to cool for at least 90 minutes.
10. Remove the cake from the pan when cool, and serve.
Note: You can also make a round version of this cake by baking in a cast iron dutch oven (oil only the bottom of the pan), at 365 degrees in a convection oven for 10 minutes (or 375 degrees in a regular oven), then another 30-35 minutes at 305 degrees. Hang upside down for several hours to cool.