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.