Whip egg yolks in mixing bowl until light and fluffy. Slowly add chocolate mixture to egg yolks, whipping constantly until fully incorporated. Set aside. Whip egg whites in another mixing bowl until foamy and add remaining .25 cup sugar. Continue whipping until stiff.
Place egg whites in bowl with chocolate and fold together. When mixture is smooth and all lumps are gone, pour into buttered and floured 12-inch spring form pan. Bake at 275 for 3 hours.
Remove from oven, let rest 20 minutes. Invert onto plate and remove form. Dust with powdered sugar and serve with vanilla sauce and/or raspberry puree, if desired.