Beat the egg in a bowl. Heat the milk and sugar on the stove until warm, then add to the egg and mix well. Sift in the rice flour, tapioca starch, and baking powder into the milk and egg mixture, stirring until smooth. Let the batter rest for 15 minutes to become smooth, or sift again if needed.

Cook the coconut with a little sugar on the stove until the coconut becomes translucent. Heat a small non-stick pot on the stove, then pour in a ladle of batter. Tilt the pot to spread the batter around the sides, then cover and cook for 2 minutes. Remove the cake and add the coconut filling.

Pour another layer of batter and place it on top of the coconut-filled cake. Alternatively, pour the batter into a pan, add the coconut, and roll it up.