Carol, this was posted in Feb of 2003 by Judy/Quebec. There are lots of other buttercream recipes on this site, but I would think a Wilton recipe would be a good place to start.

WILTON WHIPPED BUTTERCREAM ICING

"The thick but creamy texture of this flavorful icing makes it ideal for decorating. It can be refrigerated in an airtight container for up to a week. Rewhip before using."

Makes 2 cups.
Recipe may be doubled to make 4 cups, if a large or heavy dury mixer is used.

1/3 cup solid vegetable shortening
1/3 cup butter or margarine
1/4 teaspoon vanilla
3 cups sifted confectioners' sugar (icing sugar)
(there are 4 cups in a pound)
1 tablespoon plus 1 teaspoon milk

In a large mixing bowl; cream butter and shortening with electric mixer. Add vanilla. Gradually add sugar one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all the sugar has been mixed in, icing will appear dry. Add milk and beat at high speed until light and fluffy.

Keep icing covered with a damp cloth until you're ready to decorate. For best results, keep icing bowl in refrigerator when not in use. (I cover it with plastic wrap)