Gradually add sugar, beating continually until light and fluffy.
Add eggs, one at a time, beating after each addition.
Add the mashed potatoes to the creamed shortening mixture.
Add milk and flour alternately, stirring until well blended.
Chill dough for 30 minutes.
Roll out dough on a lightly floured surface to about 1/4 inch thick.
Cut with a floured doughnut cutter and let stand uncovered for 20 to 30 minutes.
Drop in batches into hot oil (375F) and fry 3-56 minutes or until golden brown. Make sure to turn as they brown.
Drain on paper towels.