In medium saucepan, melt 4 oz. chocolate and 3 tablespoons margarine over low heat, stirring constantly until smooth. Remove from heat; cool 15 minutes. Add 1/2 cup sugar and vanilla; blend well. Beat in 2 eggs 1 at a time, blending well after each addition. Add flour, baking powder and salt; stir just until blended. Pour into greased and floured pan. Pour topping over batter.