Melt chocolate over boiling water; add 1 cup sugar and 1/2 cup butterrmilk and stir over boiling water until sugar is dissolved. Cool. Sift flour once, measure, add baking powder, soda, and salt and sift together three times. Cream butter thoroughly add 3/4 cup sugar gradually, and cream together until light and fluffy. Add eggs and beat well. Add about 1/4 of flour mixture, mix thoroughly; add chocolate mixture and blend. Add remaining flour, altenately with buttermilk, a small Amount at a time; beat very thoroughly after each addition. Add vanilla. Bske in greased 15x9x2- inch pan, in a moderate oven ( 35O degrees F.) 30 minutes, or until cake is done. When cold, trim edges, cut in half crosswise, and put together as a two-layer cake, matching edges carefully. Spread tops and sides with Martha Washington's Fudge Frosting (See recipe). Or add chopped nut meats to part of frosting and use as filling.