Cream butter; gradually add 1/2 cup sugar, beating well. Add eggs, one at a time, beating well alter each addition.
Combine flour, baking powder, and cinnamon; add to creamed mixture alternately with buttermilk, beginning and ending with flour mixture. Chill dough 15 minutes.
Divide dough in half. Working with 1 portion at a time, place dough on a heavily floured surface; roll to 1/4-inch thickness. Cut dough with a floured doughnut cutter.
Heat 2 to 3 inches of oil to 350°; drop in 3 to 4 doughnuts at a time. Cook 1 minute or until golden on one side; turn and cook other side about 1 minute. Drain on paper towels. Sprinkle with powdered sugar.