A true buttercream frosting isn't cooked, and it does not contain egg -- aside from that tidbit of info -- the addition of an egg yolk gives this 'butter cream' frosting stability...and a tad more nutrition.

Cooked Butter Cream Frosting

4 to 5 tablespoons water - divided use
1 pound confectioners' (powdered) sugar - divided use
1 large egg yolk
1/4 cup butter or margarine, softened
1 teaspoon vanilla extract

1. In small saucepan, stir together 2 tablespoons of the water, 3 tablespoons of the sugar and the egg yolk. Cook over medium heat, stirring constantly, until mixture bubbles. Set aside. Allow to cool about 15 minutes.
2. In small mixing bowl, beat butter at high speed until smooth and creamy. Add 1 cup of the sugar, 2 tablespoons of the remaining water and the cooled yolk mixture. At medium speed, beat until smooth. Beat in remaining sugar, adding some of the remaining 1 tablespoon water, 1 teaspoon at a time, until of spreading consistency. Beat in vanilla. Beat at high speed until smooth, adding remaining water, if necessary.

Makes about 2 cups.

Microwave Method: In small bowl, stir together 2 tablespoons of the water, 3 tablespoons of the sugar and the egg yolk. Cook on full power stirring every 15 seconds, until bubbly, about 45 seconds to 1 minute. Set aside. Allow to cool about 15 minutes. Continue as above.

Note: All microwave cooking times are based on a full power output of 600 to 700 watts. For a lower wattage oven allow more time.

Recipe provided courtesy of the American Egg Board.