Moist, light, and fluffy donut holes tossed in cinnamon-sugar with a secret ingredient that gives them amazing texture - potato!
Make the cinnamon sugar mixture:
Place 1/2 cup sugar and 1 tablespoon cinnamon in a small bowl and toss to combine. Set aside.
Make the donuts:
Bring a saucepan of water to boil over high heat and add the potatoes with the peels still on. Cook for 20 minutes or until tender. Let potatoes cool slightly and then mash in a large bowl with skins on until no chunks remain. Measure mashed potatoes. Keep 1 cup in the bowl and discard any extra.
Add the flour, about 1 cup at a time, until a soft dough forms (I used all 4 cups, but you may need less). The dough should be just slightly sticky, but easy to roll. Do not knead the dough.
Heat 5 inches/12 cm of oil in a cast-iron skillet to 350°F (see note on how to test oil for readiness without a thermometer).*
While the oil heats, roll out dough on a floured surface about ½-inch thick. Cut out donuts with a donut cutter (I used a 2-in/5mm cutter) and carefully add a few donut holes at a time to the hot oil. To prevent oil from splashing and burning you, do not drop the donuts on the oil from more than 1 to 2 inches above the oil. Once the bottom of donuts are golden brown, turn to the other side. Drain any excess oil on a paper-towel lined plate and then immediately dredge hot donuts in cinnamon sugar mixture.
*If you don't have a thermometer, here are two ways to test the oil to see if it's ready for frying: