In a heavy saucepan, bring sugar and water to a boil; cook over medium-high heat until sugar is dissolved. Remove from the heat.
Add a small amount of hot mixture to egg yolks; return all to the pan, stirring constantly. Cook 2 minutes longer or until mixture thickens, stirring constantly. Remove from the heat; stir in vanilla. Cool to room temperature.
In a mixing bowl with the whisk attachment, cream butter until fluffy, about 5 minutes. Gradually beat in cooked sugar mixture. Beat in confectioners' sugar until fluffy, about 5 minutes.
If necessary, refrigerate until frosting reaches spreading consistency.
Store in the refrigerator.
Submitted by: Roof Raiser 101
