1. In 1-quart saucepan over low heat, melt chocolate and butter or margarine until smooth, stirring frequently. Meanwhile, grease 12" by 8" baking dish.
2. In large bowl, combine graham-cracker crumbs and walnuts; stir in undiluted condensed milk and vanilla extract until crumbs are moistened. Stir in chocolate mixture until blended. Pat brownie mixture evenly into baking dish. Let stand at room temperature about 2 hours before cutting.