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