Heat oven to 325 F.
Melt chocolate and butter in a double boiler over simmering water.
Stir sugar into chocolate until blended. Add eggs and vanilla. Stir in flour and nuts. Spread into a buttered 13" x 9" baking pan.
Bake 30-35 minutes or until toothpick inserted in center comes up with crumbs (not batter). Cool in pan.
Courtesy of Scharffen Berger Chocolate Maker