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 190ºC for 25 minutes. The center will not look done or set. Cool completely, cover and refrigerate for 4-6 hours. Cut into 4 cm squares with a hot knife. Individual pieces can be wrapped and frozen. Creme de Colorado Cookbook (1987) From the collection of Jim Vorheis