egg whites, room temperature
1In large saucepan, melt chocolate and butter together, then gradually add sugar and flour stirring constantly with wire whisk.
2Add egg yolks, one at a time, mixing well after each; Stir in vanilla.
3Beat egg whites until stiff peaks form and fold into chocolate mixture.
4Pour all but 1 cup of mixture into buttered 8x8-inch pan.
5Add 1 cup sour cream to reserved 1 cup batter and spread evenly over top of mixture in pan.
6Bake at 375 for 25 minutes; The center will not look done or set.
7Cool completely, cover and refrigerate for 4-6 hours.
8Cut into 1 1/2 inch squares with a hot knife - Individual pieces can be wrapped and frozen.