Heat oven to 350. Grease 2 square pans, 8x8x2 or 9x9x2 inches, or 3 round pans, 8 or 9x1 1/2 inches. Lines bottoms of pans with waxed paper. Pour boiling water on chocolate, stirring until chocolate is melted; cool.
Mix sugar and margarine in 2 1/2 quart bowl until light and fluffy. Beat in egg yolks, one at a time. Beat in chocolate and vanilla on low speed. Mix in flour, baking soda and salt alternately with buttermilk, beating after each addition until batter is smooth. Fold in egg whites. Divide batter among pans.
Bake until wooden pick inserted in center comes out clean. Fill layers and frost top of cake Frosting.