Butter cream: Combine cream cheese, egg, and sugar in a medium mixing bowl; beat on HI until smooth. Gradually add 2 tablespoon of milk, the butter, constarch, and vanilla extract. Add another tablespoon. of milk in needed to give smooth consistency. Set aside. Cake: Melt chocolate in top of double boiler and let cool. Cream butter and gradually add 2 cups of sugar, beating all the while. Add eggs, one at a time, beating after each addxition. Combine flour, baking powder, soda, and salt. Add to creamed mixture, alternating with the milk, beginning and ending with flour mixture. Mix each addition in well. Stir in melted chocolate and vanilla. Spread half of the chocolate batter into a greased and floured 13x9 inch pan. Spoon reserved butter cream mixture over the chocolate batter. Top with remaining chocolate batter. Bake at 180ºC until a wooden pick inserted in the center comes out clean. Let cool completely in pan on a wire rack. Frosting: Combine chocolate and butter in top of a double boiler. Bring water to a boil, reduce heat to low and cook and stir until chocolate and butter has melted. Remove from heat. Let cool. Add confectioner's sugar and half of the milk, beating at medium speed. Add the vanilla and additional milk as needed to bring to spreading consistency. Spread on cooled cake.