Prep the potatoes. If you don’t have leftover mashed potatoes, boil peeled potato chunks until tender. Mash until smooth and let cool completely. Avoid adding butter or milk; you want a neutral mash.

Mix dry ingredients. In a large bowl, whisk flour, sugar, baking powder, baking soda, salt, nutmeg, and cinnamon. This evenly distributes leaveners for a consistent rise.

Whisk wet ingredients. In another bowl, combine eggs, buttermilk, melted butter, vanilla, and mashed potatoes. Whisk until mostly smooth. A few small potato bits are okay, but keep it lump-light for even texture.

Bring it together. Add the wet mixture to the dry ingredients. Stir with a spatula until a soft dough forms. It should be tacky but not soupy. If it’s too sticky to handle, add 1–2 tablespoons of flour at a time.

Rest the dough. Cover the bowl and let it sit for 10–15 minutes. This rest hydrates the flour and relaxes the dough, making it easier to roll and helping the donuts stay tender.

Roll and cut. Lightly flour your counter. Pat or gently roll the dough to about 1/2 inch thick. Use a 3-inch donut cutter (or a round cutter and a small cap for the hole). Gather scraps, rest them 5 minutes, then re-roll once.

Heat the oil. In a heavy pot, heat 2–3 inches of oil to 350–360°F (175–182°C). Keep a thermometer clipped to the pot for accuracy. Prepare a wire rack set over a baking sheet.

Fry in batches. Carefully lower 2–4 donuts at a time into the oil. Fry about 1–1 1/2 minutes per side until deep golden brown. Keep oil temp between 350–360°F. Adjust heat as needed.

Drain and finish. Transfer donuts to the rack to drain. While warm, toss in cinnamon sugar or let cool slightly and dip in glaze. If using powdered sugar, dust after the donuts are fully cool.

Fry the holes. Donut holes cook fast—about 30–45 seconds per side. They make great testers for oil temperature and seasoning.