Melt together butter and baking chocolate in a pan on low heat. Add the powdered sugar, vanilla, and egg yolks. Add 3/4 c. chopped walnuts. Beat 2 egg whites until stiff -- fold into chocolate mixture which has cooled slightly. Crush vanilla wafers to make 1-1/2 cups. Cover bottom of 13" x 9" x 2" pan with half of crumbs. Add chocolate mixture and freeze for at least 2 hours. Remove and spread ice cream on top. Sprinkle with remaining crumbs. Freeze again until ready to serve. Can keep in freezer for up to a week covered with foil.