Peel and cut the purple sweet potato and taro into evenly sized-chunks. Steam them for 20-30 minutes, or until they are soft enough to be easily pierced with a fork.

Once steamed, place the taro and purple sweet potato into a blender or food processor. Add the condensed milk, sugar, and butter or corn oil. Blend until smooth and creamy. Transfer the taro filling into a piping bag and set aside.

Mix the Batter: In a mixing bowl, combine the glutinous rice flour, cornstarch, granulated sugar, and milk. Stir until smooth. To achieve a finer texture, strain the mixture through a sieve.

Prepare for Steaming: Cover the mixture with plastic wrap, making sure to poke a few small holes with a toothpick to allow steam to escape. This helps prevent water droplets from dripping into the dough.

Steam: Place the covered mixture in a steamer and steam over medium-high heat for 25 minutes. After steaming, add the butter to the hot dough. Using a silicone spatula, thoroughly mix until the butter is fully incorporated.

Knead the Dough: Once the dough has cooled slightly and is safe to handle, put on gloves and knead the dough for about 5 minutes. Stretch and pull the dough until it becomes elastic and smooth. This step is crucial for achieving the perfect mochi texture. Dust your hands and the dough with some cooked glutinous rice flour to prevent sticking. Divide the dough into roughly 50-gram portions and set aside.

Take one portion of mochi dough and roll it into a ball. Then, flatten the dough into a small disc.

Pipe a small amount of taro filling onto the center of the dough. Gently wrap the dough around the filling, pinching the edges to seal it. Roll it between your palms to shape it into a smooth ball.

Roll the finished mochi in coconut flakes to coat the outside, adding a pleasant texture and flavor.