This frosting is made the old-fashioned way — on the stovetop.
In large mixing bowl, beat the egg yolks until light in color and tripled in volume, about 7 minutes.
In a small saucepan, combine the sugar and water. Cook over medium heat, stirring constantly until the sugar dissolves, about 2 minutes. Increase the heat to medium high and bring the mixture to a boil. Continue to boil, without stirring, until the mixture reaches soft-ball stage (238°F on a candy thermometer).
Remove the pan from the heat and pour a small amount of sugar syrup over the egg yolks. Beat on high speed and continue to add sugar syrup. Continue beating for about 5 minutes or until mixture has cooled.
Lower the speed and begin adding the butter a tablespoon at a time. Add the vanilla and continue beating until spreading consistency.
Makes about 4 cups.
This ultimate buttercream is so buttery and creamy, use only the freshest butter, and please, no substitutes.