Prepare the taro by peeling, washing, and slicing it into cubes.
Peel and chop the purple potato.
Steam the chopped taro and purple potato until soft enough to mash.
After steaming, blend the steamed taro and purple potato in a food processor or blender. Blend until smooth like paste.
In a non-stick pan, toast the glutinous rice flour over low heat until its white color turns slightly cream-colored.
In a bowl, mix and knead the steamed taro and purple potato, cooked glutinous rice flour, sugar, and milk powder. The quantity of cooked glutinous rice flour is not fixed; add it gradually in small amounts until the mixture forms a cohesive dough.
Divide the mooncake dough into 40g portions.