Combine mashed potatoes, butter, sugar, eggs, vanilla, and buttermilk. Beat until well blended. Sift together flour, baking powder, baking soda, salt and nutmeg. Gradually stir dry ingredients into potato mixture, blending well. Chill 1 hour. Roll out to 1/2 inch thick and cut with donut or biscuit cutter. Drop in hot oil at 360 degrees and cook until golden on one side then flip and cook until golden on other. Drain on paper towel. Roll in cinnamon sugar or powdered sugar.
Makes about 6 dozen donuts