Cream butter in a large mixing bowl; gradually add sugar, beating well. Add eggs; beat well. Set aside.
Place chocolate in top of a double boiler; bring water to a boil. Reduce heat to low, and cook until chocolate melts; let cool slightly.
Sift together flour and salt. Add flour mixture and chocolate to creamed mixture; mix well. Stir in vanilla and walnuts.
Pour batter into a waxed paper-lined and greased 8-inch square baking pan. Bake at 400° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Frost and cut into 2-inch squares to serve.