Wash and peel the purple sweet potato, then slice it into medium pieces. Steam until fully cooked, then mash while still hot until smooth.

In a mixing bowl, combine the cake flour, tapioca starch, salt, and granulated sugar. Mix well. Pour in the yeast mixture and mix again. Add the mashed purple sweet potato and knead until the dough is smooth and elastic.

Cover the bowl tightly with plastic wrap and let the dough rise for 40 minutes or until doubled in size. Meanwhile, gently heat the coconut milk until just warm (about 95°F/35°C). Once the dough has risen and is fluffy, add the warm coconut milk and coconut oil. Mix thoroughly, cover again with plastic wrap, and let rise for another 30 minutes.

When the batter is full of bubbles and very airy, stir it well and prepare for steaming. You can use cups, small bowls, or any molds you have. Grease them lightly with oil to prevent sticking.

Bring water to a boil in a steamer over high heat. Pour the batter into the prepared molds—fill them to the top for a cracked, domed look (no need to preheat the molds). While steaming, occasionally release the steam and wipe the lid dry to prevent water from dripping onto the cakes. Steam until cooked through, about 20–40 minutes depending on mold size. To check for doneness, insert a toothpick—if it comes out clean, the cake is done.

Remove the molds from the steamer and let cool. Take the cakes out and enjoy. The honeycomb cake will have a beautiful purple color, a gentle sweetness, a soft, chewy, fluffy texture, and a wonderful aroma from the sweet potato.