Classic Buttercream Frosting

Classic Buttercream Frosting
(adapted from "Neoclassic Buttercream" Cake Bible pg.230)
Yield about 4 cups
Prep time: 5 minutes  Total Time: 15 minutes

Ingredients:
6 large Egg Yolks
3/4 cup Sugar
1/2 cups Corn Syrup
3/4 lb Softened Butter (3 Sticks)
1/2 cup Shortening
2-4 tablespoons flavoring (optional)

Method of Preparation:
1.  In a bowl beat the yolks with an electric mixer until light in color.   Meanwhile, combine the sugar and corn syrup in a small saucepan and heat, stirring constantly, until the sugar dissolves and the syrup comes to a rolling boil. ( The entire surface will be covered with large bubbles.) Immediately transfer the syrup to a greased 1-cup heatproof glass measure near the range.
2.  If using an electric hand-held mixer, beat the syrup into the yolks in a steady stream.  Don't allow the syrup to fall on the beater or they will spin it onto the sides of the bowl.  If using a stand mier, pour a small amount of syrup over the yolks with the mixer turned off. Immediately beat at high speed for 5 seconds. Stop the mier and add a larger amount of syrup.
3.  Beat at high speed for 5 seconds.  Continue with the remaining syrup.  For the last addition, use a rubber scraper to remove the syrup clinging to the glass measure.  Continue beating until completely cool.
4.  Gradually beat in the butter and shortening and, if desired, any optional flavoring.  Place in airtight bowl.  Bring to room temperature before using. Rebeat to restore texture.