All ingredients should be at room temperature. Preheat oven to 350 degrees F. Butter and flour two 9" cake pans.
In a double boiler, stir together and melt 4 ozs. of chocolate, 1/2 cup of the milk, brown sugar and one egg yolk, stirring occasionally. Set this mixture aside to cool slightly. Before measuring, sift the cake flour. Resift with the baking soda and salt. Cream butter and add the granulated sugar, beating until light. Beat in the last 2 yolks one at a time. Blend together water, 1/4 cup of the milk, sour cream and vanilla. Add flour mixture to the butter mixture, in 3 parts, alternately with the liquid ingredients. Stir batter until smooth, then blend in the chocolate which was set aside. Fold in 2 beaten egg whites. Bake 25 minutes.
