A tasty recipe for mashed potato donuts that I found on Taste of Home (tweaked slightly); they are fluffy on the inside and crisp on the outside.
A good way to use up leftover mashed potatoes.
NOTE: mashed potatoes include milk and butter
In a large bowl, beat eggs and sugar. Add the potatoes, buttermilk, butter and vanilla. Combine the flour, baking powder, baking soda, and salt; gradually add to egg mixture and mix well. Cover and refrigerate for 1/2 an hour to two hours.
Turn onto a lightly floured surface; roll to 1/2-in. thickness. Cut with a floured 2-1/2-in. doughnut cutter OR flour your hands and shape dough into donut shape.
In an electric skillet or deep-fat fryer, heat oil to 375°. Fry the doughnuts, a few at a time, until golden-dark brown on both sides; making sure they don't burn, about 2 minutes. Drain on paper towels. Roll the warm doughnuts in sugar/brown sugar combo or confectioners sugar if desired. Yields: about 1 dozen.