Makes about 2⅓ cups

In a medium-sized metal bowl, whisk together the sugar, egg yolks and water. Set the bowl over a saucepan of simmering water. Do not allow the bottom of the bowl to touch the water. Whisk constantly until the mixture registers 170° on a candy thermometer, about 4 minutes.

Remove the bowl from over the water. Using an electric mixer set on high speed, beat the egg mixture until cool and thick, about 5 minutes. Gradually add the butter, about 1 tablespoon at a time, beating until smooth after each addition. Beat in the vanilla extract. If the buttercream appears broken or lumpy, set the bowl back over simmering water for a few seconds, then beat again until smooth.

To store, cover and refrigerate for up to 2 days. Before using, let stand at room temp until softened. If necessary, rewarm over a saucepan of simmering water for a few seconds, the beat until smooth.