In the bowl of a standing mixer, beat the eggs and egg yolks until light and fluffly and doubled in size.

Combine the sugar and corn syrup in a small saucepan over medium heat, stirring constantly until the sugar dissolves and they syrup comes to a rolling boil. Transfer the syrup to a heat proof bowl or measuring cup to stop the cooking.

Pour a small amount of the syrup into the egg yolks begin beating with the paddle attachment on low speed. Pour the remaining corn syrup into the egg mixture in a slow, steady stream. Continue beating until the completely cool and fluffy, about 5 minutes. Gradually beat in the chocolate, butter and vanilla until the icing is is light and fluffy, about 3 minutes.

Use immediately or cover and refrigerate for up to 1 week. Let the buttercream come to room temperature before using.