Toss together wafer crumbs and 4 tablespoons melted butter. Reserve 1/4 cup of the mixture. Press remaining wafer crumbs in a 9 x 9-inch baking pan. Spread with ice cream; freeze. Melt 1/2 cup butter and the chocolate over low heat; gradually stir into egg yolks with the powdered sugar, nuts and vanilla. Cool thoroughly. Beat egg whites until stiff. Beat chocolate mixture until smooth. Fold in egg whites. Spread chocolate over ice cream. Top with reserved wafer crumbs. Freeze. Makes 8 servings. Can be doubled in a 9 x 13-inch pan.