Preheat the oven to 300 degrees F. Butter and flour a 12-inch springform pan. In a double boiler, over medium heat, melt the chocolate and butter together. Stir in the Grand Marnier. Remove from the heat and cool to room temperature. In an electric mixer, on medium speed, beat the egg yolks until pale yellow and ribbon like, about 6 to 8 minutes. During the last 2 minutes, add the sugar in a steady stream. With the machine running slowly add the chocolate mixture and blend until all of the chocolate is incorporated. Fold in the stiff each whites. Pour the batter into the prepared pan. Place the pan on the rack, in the center of the oven. Bake the cake for about 1 1/2 to 2 hours or until the center of the cake comes out clean with a toothpick. Remove the cake from the oven and place on a wire rack. Cool the cake for 15 minutes. Remove the sides of the pan and cool completely. Invert the cake onto a serving plate. The cake can be served warm or can be placed in the refrigerator and chilled. Slice the cake into 16 slices and serve the cake with the white chocolate sauce, whipped cream and mint.