Melt chocolate with butter in saucepan over very low heat; stirring constantly until smooth. Cool slightly. Beat eggs, sugar, and vanilla in a large mixing bowl at high speed 10 min. Blend in chocolate at low speed. Add flour, beating just to blend. Stir in walnuts. Spread in greased 13x9 inch pan. Bake at 190ºC for 35-40 minutes. (do not overbake). Cool in pan. Cut into bars or squares. Makes 24-32