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.
Best results are obtained when butter and sugar are at room temperature
and egg whites and milk are cold out of the refrigerator.