Cream butter and sugar. Blend in mashed potatoes, then eggs. Sift dry ingredients together twice. Add to first mixture alternately with milk. Chill dough 2 hours. Roll out 1/2-inch thick on lightly floured surface. Cut with doughnut cutter. Fry in deep hot fat at 360° about 1 minute on each side. Fry one layer at a time and turn once. Drain on unglazed paper. Sprinkle with confectioners sugar while warm.