In a large mixing bowl cream together the butter and sugar. Heat the milk in a small saucepan without boiling. In a double boiler over barely simmering water, melt the chocolate. In a medium mixing bowl whisk together the egg yolk and milk. Pour the melted chocolate into the milk mixture, whisking constantly. Stir into the creamed butter mixture, mixing well. Transfer the mixture to a large pastry bag fitted with a 1/4-inch plain pastry nozzle. In a small bowl stir together the cognac and water. Turn half the macaroons baked-bottom side up and brush lightly with the cognac mixture. Pipe1/2 teaspoon of filling into the macaroons. Place plain macaroons, dome side up, on top of the filled ones. Press gently to fit each pair together.
