Place egg yolks in medium-size mixing bowl.
Add confectioners sugar; blend well with wooden spoon.
Place bowl over hot water at medium heat; stir until mixture is very thick and creamy and free from streaks.
Remove from hot water.
Place bowl in pan of ice; stir until cool.
Cream butter until smooth; whip into sugar mixture a spoonful at a time with electric mixer at medium speed until well blended.