Begin by setting out all ingredients several hours before beginning, so everything is at room temperature.
Have ready a greased 1-cup heat-proof glass measuring cup near your stove.
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 (do not use a non-stick sauce pan). Heat, stirring constantly, until the sugar dissolves and the syrup comes to a roiling boil. Immediately transfer the syrup to the prepared glass measuring cup to stop the cooking.
If using a hand-held mixer, beat the syrup into the yolks in a thin, steady stream. Don't allow syrup to fall on the beaters or they will spin it onto the sides of the bowl. If using a stand mixer, pour a small amount of syrup over the yolks with the mixer turned off. IMMEDIATELY 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 measuring cup. Continue beating until completely cool.
Gradually beat in the butter and flavoring. Place in an airtight bowl.
Will keep for 6 hours at room temperature, 1 week refridgerated, or 8 months frozen. Be sure to bring to room temperature completely before rebeating to restore texture, or the frosting may curdle.