In a saucepan, combine chocolate, 1/2 cup milk, brown sugar, and egg yolks. Cook over low heat, stirring constantly, until custard is thick and smooth. Cool.
Stir together flour, soda, baking powder, and salt.
In a large bowl, cream butter or margarine. Blend in white sugar, beating until light and fluffy. Mix together 3/4 cup milk and vanilla. Stir flour mixture into creamed mixture alternately with flavored milk. Make 3 dry and 2 liquid additions, combining lightly after each. Stir in cooled chocolate custard.
Beat egg whites to form stiff but moist peaks. Fold into cake batter, and beat by hand for 1 minute. Turn batter into two greased and floured 8 inch round cake pans.
Bake at 350 degrees F (175 degrees C) for 25 to 30 minutes, or until cake springs back when lightly touched. Cool layers for 5 minutes, and then remove from pans. Cool completely.
