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 F 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.
Creme de Colorado Cookbook (1987) From the collection of Jim Vorheis