Cream 4 tablespoons butter. Add 3/4 cup confectioners' sugar and blend. Add 1 teaspoon vanilla, 2 squares unsweetened chocolate, melted and 1/4 teaspoon salt. Beat 2 egg whites until stiff but not dry. Add 3/4 cup sifted confectioners' sugar, 2 tablespoons at a time, beating after each addition until blended. Then continue beating until mixture will stand in peaks. Add to chocolate mixture, folding in gently but thoroughly enough to blend.