In a large bowl, beat the potatoes, eggs, sugar, buttermilk, butter and lemon zest until blended. Combine the flour, baking powder, salt, nutmeg and baking soda; gradually beat into potato mixture and mix well., Turn onto a lightly floured surface; roll to 1/2-in. thickness. Cut with a floured 2-1/2-in. doughnut cutter. In a deep cast-iron or electric skillet, heat oil to 375°. Fry doughnuts and doughnut holes, a few at a time, until golden brown on both sides. Drain on paper towels. Combine sugar and cinnamon; roll warm doughnuts in mixture.