2 cups butter or margarine
2 cups sifted confectioners' sugar
2/3 cup sweetened condensed milk
2 large egg whites
1 cup sifted confectioners' sugar, divided
1/2 teaspoon salt
1/2 teaspoon vanilla extract
Place butter in mixing bowl, add 2 cups confectioners' sugar, mix at low speed to obtain smooth paste. Whip at medium speed, adding the sweetened condensed milk slowly and gradually until light and fluffy.
Using a very clean bowl and beater, whip the egg whites until stiff while adding 1/2 cup confectioners' sugar slowly. Mix this meringue slowly into the above butter mixture. Add vanilla extract and remaining 1/2 cup confectioners' sugar. This last amount of sugar can be doubled if stiffer icing is desired.
Note: Best results are obtained when butter and sugar are at room temperature and egg whites and milk are cold out of the refrigerator.