To make the purple sweet potato batter:
- In a bowl, combine 60 grams sugar (about 1/4 cup), 100 ml warm water (about 1/3 cup plus 1 tablespoon), 100 grams tapioca starch (about 3/4 cup), vanilla, 150 grams coconut milk (about 2/3 cup), and 150 grams mashed purple sweet potato (about 5.3 oz). Mix well. Strain the mixture through a sieve for a smooth batter.

To make the white coconut batter:
- In another bowl, combine 60 grams sugar (about 1/4 cup), 80 ml warm water (about 1/3 cup), 170 ml coconut milk (about 3/4 cup), vanilla, and 150 grams tapioca starch (about 1 1/4 cups). Stir well, strain through a sieve, and let the batter rest for 15 minutes.

Prepare a steamer with plenty of water. Brush a thin layer of vegetable oil around the inside of the cake mold and place it in the steamer. Pour in a layer of purple sweet potato batter. Steam for about 3–4 minutes until set, then pour a layer of coconut batter over the cooked layer and continue steaming until set. Repeat, alternating layers, until all the batter is used or the mold is full.

Once the final layer is cooked, remove the mold from the steamer and let the cake cool completely. Run a sharp knife around the edge to loosen, then invert onto a plate to remove the cake. The purple color is beautiful, and the cake is fragrant and delicious—everyone will love it.

The purple color is beautiful, and the cake is fragrant and delicious—everyone will love it.