4 egg yolks
1/3 cup sugar
12 Tbs. unsalted butter, at room temperature
1/2 tsp. vanilla
In the top pan of a double boiler or a heatproof bowl placed over but not touching simmering water, combine the egg yolks, sugar. Whisk until very thick and pale, about 4 minutes. Remove the pan or bowl from over the water and let cool to room temperature.
In a bowl, combine the butter and vanilla. Using an electric mixer set on medium-high speed, beat until very soft and fluffy, about 5 minutes. Gradually beat in the cooled egg yolk mixture. Let the mixture stand at room temperature, stirring occasionally, until spreadable, about 20 minutes.
Use the frosting immediately, or cover and refrigerate overnight or freeze for up to 1 month. Before using, let stand at room temperature until softened, then beat with an electric mixer set on high speed until smooth and fluffy. Makes about 1 1/2 cups.