Melt unsweetened chocolate and butter in top of double boiler over hot water. Cool slightly. Gradually add sugar to eggs, beating thoroughly after each addition. Blend in chocolate mixture. Stir in flour. Add vanilla, half of nuts and half of semisweet chocolate pieces. Spread in greased 23 cm square pan. Sprinkle remaining nuts and chocolate pieces over batter. Bake at 160ºC 30 minutes. Remove and cool in pan. Cut in squares.