Cream butter in a large mixing bowl; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition. Add chocolate, beating well.
Combine flour, baking powder, and soda; add to creamed mixture alternately with boiling water, beginning and ending with flour mixture. Beat well after each addition. Gently stir in vanilla.
Pour batter into a greased and floured 13- x 9- x 2-inch baking pan. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes. Turn out on a flat surface, and cool completely. Spread Dark Fudge Frosting on top and sides of cake.