In a large bowl, beat the yolks with an electric mixer until light in color; reserve.
Combine the sugar and corn syrup in a small saucepan (preferably nonstick) over medium heat, stirring constantly, until the sugar dissolves and the syrup comes to a rolling boil. Transfer the syrup to a heatproof bowl to stop cooking. Pour a small amount of the syrup into the yolks and begin beating with the electric mixer. Pour the remaining corn syrup into the mixture in a steady stream. Continue beating until completely cool and fluffy, about 5 minutes. Gradually beat in the butter and vanilla until the icing is light and fluffy, about 3 minutes. Use immediately. (Or cover and refrigerate for up to 5 days.)
Makes about 6 cups