Preheat oven to 325 degrees. Melt chocolate and butter together in microwave on high for 1 to 2 minutes, or on stove top in heavy saucepan on very low heat. Stir until chocolate is completely melted. Stir sugar into chocolate until well blended. Mix in eggs and vanilla. Stir in flour and nuts until well blended. Spread in greased 9x13-inch glass baking dish. Bake 30 minutes. Do not overbake. Cool in pan. Cut into squares.