The drier your potatoes are, the lighter your doughnuts will be, so we bake or microwave them, rather than boil them. Running the cooked spuds through a potato ricer gives a fluffier texture. When it comes to frying doughnuts, we find that melted vegetable shortening produces a crisper crust. Feel free to just use leftover mashed potatoes, though, provided they aren’t flavored with garlic, chives, etc.