Preheat oven to 350 degrees. Grease 2 - 9 inch layer cake pans. Whisk together chocolate, milk, allspice, brown sugar and one egg yolk in the top of a double boiler, stirring until custard is formed and mixture thickens. Cool. Stir in baking soda and salt. Sift the flour onto waxed paper. In bowl of electric mixer, whip the butter, gradually adding sugar and two egg yolks, one at a time, beating well after each addition. In a cup, stir together 3 tbsp. water, 1/2 cup milk, and 1 tsp. vanilla extract and 1 tbsp. dark rum. Add the flour mixture to the butter mixture alternating with the vanilla/milk/water mixture. Stir in the chocolate custard mixture and beat until smooth, approximately two to three minutes at medium speed of mixer. In a clean bowl, using the wire whisk attachment of your electric mixer beat two egg whites until soft peaks form; gently fold into cake batter. Turn into greased layer cake pans. Bake at 350 degrees. for about 25 minutes or until cake tester or toothpick inserted in center comes out clean. Turn out onto wire racks to cool. Level with a serrated knife if required, then stack and frost with chocolate frosting.