Mix well dry yeast with warm milk and sugar. Wait the yeast until active (until the yeast show the foam), meanwhile you can mash all boiled potato.
Mix well the 3/4 flour, activated yeast, eggs, and the mashed potato in the large bowl with mixer in medium speed.
Put the salted butter to the dough. Mix well until the dough is glossy, smooth, and elastic when pulled. You can use mixer in high speed about 20 mins, or using your own hand by kneading it.
Wrap the bowl with plastic/cling wrap/napkin about 30 mins until the dough becomes doubled size.
Unwrap the dough and cut it as you like. Roll the dough piece into smooth and tight texture. Your dough is ready to cut with donut ring.
Place the dough piece in the floured tray so it does not stick each other.
Fill your deep fried pan with vegetable oil. When the oil is heated, you can start to fry the dough until it becomes golden brown color.
Toss the donut with caster sugar until it full covered while still warm. Now your snow sugar donut with potato are ready to serve.