Sift together flour, sugar, cocoa, baking soda, baking powder, and salt in a mixing bowl. Add eggs, oil, coffee, buttermilk and vanilla. Beat with an electric mixer set at medium speed for 3 minutes. Pour batter into greased 13x9 inch baking pan. Bake in preheated 350 degree oven for 40 minutes or until the cake tests done.
Cool in pan on rack. Frost with Hundred Dollar Frosting. Cut into squares.
HUNDRED DOLLAR FROSTING: Combine butter and semisweet choclolate in a double boiler top. Place over hot water, stirring until melted. Remove and cool well. Add egg and stir vigorously. Stir in confectioners sugar, vanilla and lemon juice. Beat until smooth. Stir in walnuts and frost cake with icing.