Notes: You can make this frosting up to 1 week ahead; cover airtight and chill. Before using, let buttercream come to room temperature, then beat briefly at medium speed to restore texture. In place of the vanilla in step 4, you can flavor buttercream with 2 to 3 tablespoons liqueur, 6 ounces bittersweet chocolate, melted, or up to 1/2 cup strained jam of any flavor. Soften the butter briefly in in a warm place in the kitchen or in a microwave oven until very soft but not melted.
In a bowl, with a mixer at high speed, beat egg yolks until pale yellow, 4 to 5 minutes.
Butter a 2-cup glass measure. In a 2- to 3-quart pan, combine sugar and corn syrup. Set over medium-high heat and stir until sugar is dissolved and mixture comes to a rolling boil. Immediately pour into the buttered glass measure.
Beating constantly at medium speed, pour syrup in a thin, steady stream into egg yolks (avoiding beaters). Continue beating until mixture is at room temperature, 7 to 10 minutes.
Add butter and vanilla. Beat just until smooth.
Nutritional analysis per 1/4 cup.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.