Cream butter in a large mixing bowl; gradually add sugar, beating well. Add eggs; beat until well blended.
Dissolve soda in buttermilk; set aside.
Combine flour, cocoa, baking powder, and salt; sift 3 times. Add to creamed mixture alternately with reserved buttermilk mixture, beginning and ending with flour mixture; mix well. Stir in vanilla.
Pour batter into a greased and floured 9-inch square baking pan. Bake at 375° for 25 minutes or until a wooden pick inserted in center comes out clean. Remove cake from pan; cool on a wire rack.
Cut cake into 2-inch squares. Spread top and sides of each square with Seven-Minute Frosting. Garnish each with chocolate morsels to resemble dice.