Melt butter or margarine and chocolate in a saucepan. Remove from heat; stir in 1 cup sugar. Blend in eggs, one at a time. Add vanilla. Stir in flour and nuts and mix well. Spread in greased 8 X 8 X 5 cm baking pan. Bake at 180ºC or 30 minutes. Cool; cut into squares. Makes 16 brownies.