Beat the egg until light. Beat in the sugar. Melt the chocolate and butter together. Add to the egg mixture. Stir in the mashed potatoes. Mix the flour, salt and baking powder together. Add the milk to the egg mixture, alternating with the flour mixture. Chill for 1 hour. Roll out on a lightly floured board to a 3/8" thickness. Cut using a floured doughnut cutter. Fry until crusty brown (about 2 minutes). Drain. Prepare the frosting by melting the butter and chocolate together. Beat in the sugar, boiling water and vanilla. Mix thoroughly. Coat the doughnuts with the frosting after they have cooled.