TO MAKE THE CAKE: Preheat oven to 180ºC Buter and lightly flour 2 8 inch round cake pans. Place the cocoa, 3 T of sugar and the water in a small pan and cook over low heat until smooth and blended. Remove from the heat and stir in the milk; set aside. Cream the butter, add the vanilla and half cup of sugar and beat until light. Beat in the yolks and gradually add the cocoa mixture. Mix well. Mix the flour, cream of tartar, salt and baking soda, add to the first mixture and blend until just smooth. Beat the egg whites until foamy. Slowly add the remaining sugar and beat until the whites are stiff but not dry. Fold the whites into the batter. Spread the batter into the prepared pans and bake until a toothpick inserted into the center comes out clean, about 25 to 30 minutes. Cool in the pans for 5 minutes before turning out onto wire racks. FROSTING: Stir together all the ingredients except the vanilla. Bring to a rolling boil and cook, stirring vigorously for 1 minute. Let cool completely. Add the vanilla and beat until thick. Frost the cake. SOURCE: Molly O'Neill in the NY Times Magazine 0. 175438596491.23 Adapted from "The Fanny Farmer Cookbook" by Marion Cunningham, Alfred A, Knopf, 1990 ----