Inspired by Amei Tea House in Jiufen, Taiwan, we used orange and purple sweet potatoes (not to be confused with purple yam), and moon cake molds. The cakes are a mixture of mashed sweet potatoes, toasted rice flour, salt and melted butter. How much flour you need to add depends on how sticky your mashed sweet potato is. Start with the minimum and add the rest as needed. As a guide, the mixture should not stick to your hands when you roll it into balls.
Place the cleaned sweet potatoes in a pot and cover with water. Bring to the boil, lower the heat, cover the pot and simmer for about 20 minutes or until a knife inserted at the center goes through without resistance.
Cool the sweet potatoes.
While the sweet potatoes cool, toast the rice flour in an oil-free pan until lightly browned. Cool.
Peel the sweet potatoes and mash. You will need two cups of mashed sweet potatoes.
Add the salt and rice flour to the sweet potatoes. Mix lightly but thoroughly. Overmixing will make the mixture sticky and lumpy.
Taste the sweet potato mixture. If you want to sweeten it some more, add sifted powdered sugar, no more than a tablespoon at a time.
Form the sweet potato mixture into small balls.
Press each ball into the mold then use the handle to release.