Melt chocolate in a double boiler over hot water. Add condensed milk and cook, stirring constantly, about 5 minutes or until mixture thickens. Add vanilla, graham cracker crumbs and one cup nuts; mix well. Sprinkle 1/4 cup of remaining nuts in the bottom of a buttered 8"x8" baking pan. Use a knife that has been dipped in hot water to spread chocolate mixture in pan. Press remaining 1/4 cup nuts into top. Cover and chill overnight. Cut into squares.