In a mixing bowl, cream butter and sugar. Add eggs and vanilla; mix well. Combine flour, cocoa and walnuts; add to the creamed mixture. Spread into a greased 13x9x2-inch baking pan. Bake at 350° for 30 minutes or until a toothpick inserted near the center comes out clean. Combine condensed milk and coconut; carefully spread over the hot chocolate layer. Bake at 350° for 20 minutes or until coconut is lightly browned. Combine icing ingredients until smooth; spread over warm bars. Refrigerate at least 1 hour before cutting.