Melt 3 squares chocolate and 1/2 cup margarine together over low heat. Set aside. Beat eggs until foamy; add sugar and beat well. Add chocolate mixture, beating until smooth. Add flour and salt. Beat. Add vanilla. Beat. Stir in chopped nuts, if desired.
Pour into greased glass 9x13-inch pan. Bake at 325 degrees for 30 minutes. Frost carefully while hot.
For frosting: Melt 1 square unsweetened chocolate and 1 tablespoon margarine together over low heat. Add skim milk and powdered sugar. Add more sugar if needed to make smooth frosting.