Preheat the oven to 375 degrees F. Grease a 9 inch springform pan. Using an electric mixer, fitted with a paddle, cream the butter and 1 cup of sugar together. With the machine running add the egg yolks and beat until incorporated. Sift the flour, soda, baking powder, and salt together. Add the sifted ingredients to the creamed butter in 3 parts, alternating with thirds of the sour cream. Beat until the batter is smooth. Add the vanilla. Fold in the beaten egg whites into the batter. Pour the batter into the prepared pan and place in the oven. Bake for about 25 minutes, or until the center comes out clean. Remove the cake from the oven and cool on a wire rack. After the cake has cooled, use a knife to loosen the sides from the springform pan. Remove the springform and slice. In a mixing bowl, combine the remaining sugar with the berries. Using the back of a fork, lightly mash the berries. Add the Grand Marnier to taste. To serve, place a slice of the cake in the center of each plate. Spoon some of the macerated berries over piece of cake. Garnish the slice with whipped cream, powdered sugar and mint sprigs.