center of cake. Cool slightly then remove layers from pans and cool completely on wire rack. To make frosting, combine 3/4 cup sugar and brown sugar in heavy saucepan. Add hot water and cream of tartar and stir to blend. Cover pan and slowly bring to simmer. Uncover pan and cook until syrup spins long thin thread. Remove from heat. Beat 3 egg whites until stiff. When syrup stops bubbling, pour over beaten whites in thin stream, beating constantly until fluffy. Use to frost cooled layers. If desired, melt 1 ounce semisweet chocolate with 1/4 teaspoon butter and drizzle over frosting. Carefully swirl it over frosting with a thin spatula to give marbled effect.