Preheat oven to 375°. Lightly grease 2 (9-inch) cake pans. In the top of a double boiler, combine the chocolate, 1/2 cup milk, 1 cup brown sugar and 1 egg yolk. Keep below boiling point, but cook until thick and smooth. Set aside to cool. Sift the granulated sugar and beat the butter until smooth. Add the sugar and blend until light and creamy. Beat in 2 egg yolks, one at a time. Before measuring, sift the flour and then resift with baking soda and salt. Combine 1/4 cup water, 1/2 cup milk and the vanilla. Add the flour to the butter in 3 parts, alternating with thirds of the liquid. Stir in the chocolate. Whip 2 egg whites until stiff but not dry. Fold them lightly and carefully into cake batter. Pour the batter into the greased cake pans and bake for 25 minutes. Top with white icing.