Preheat the oven to 350 degrees F. Coat the bottom and sides of an 8'x8"x2" baking dish with the butter. Set aside. Combine the sugar, flour, and baking powder - sifting them together into a bowel. Make a well in the center and drop in the egg. Mix together until the egg is absorbed into the flour. In a large bowel, mix together the mint and the cinnamon. Add the apples and toss until the apples are coated. Arrange the slices in the baking dish and scatter the flour mixture over them, spreading and pressing gently to cover the apples completely. Bake in the oven center for 45 minutes, or until the top is crusty. Serve at room temperature accompanied by the whipped cream.