Cream shortening; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition. Add mashed potatoes; beat well.
Sift together flour, baking powder, salt, and spices; add to creamed mixture alternately with milk, beginning and ending with flour mixture. Chill dough at least 1 hour.
Turn dough out onto a heavily floured surface; roll to 1/3-inch thickness. Cut dough with a floured doughnut cutter.
Heat 3 to 4 inches of oil to 375°; drop in 3 or 4 doughnuts at a time. Cook 1 minute or until golden brown on one side; turn and cook other side. Drain on paper towels. Sprinkle with powdered sugar.