Source: Star Chefs
From The Cake Bible by Rose Levy Beranbaum (William Morrow & Co., 1988)
Adapted by StarChefs.com
Yield: 4 cups
6 large egg yolks
3/4 cup sugar
1/2 cup corn syrup
2 cups unsalted butter, softened
2 to 4 Tablespoons liqueur or eau-de-vie of your choice
Have ready a greased 1 cup heatproof glass measure near the range.
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 (preferably with a nonstick lining) 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 the glass measure to stop the cooking.
If using an electric hand-held mixer, beat the syrup into the yolks in a steady stream. Don’t allow syrup to fall on the beaters or they will spin it onto the sides of a 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. Stop the mixer and add a larger amount of syrup. 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.
Gradually beat in the butter and, if desired, any optional flavoring. Place in an airtight bowl. Bring to room temperature before using. Rebeat to restore texture.
UPDATE: I made this buttercream as directed, and I did not like it. I wanted vanilla buttercream, so I added vanilla extract - but it still tasted like softened butter. I think "regular" buttercream is still too sweet, but this isn't sweet enough. I'm looking for something in between. I had to add confectioner's sugar to this to make it sweeter, which then loses the soft smooth texture, which I did like. I wonder if it would be better if I made more syrup, but used the same amount of butter. Back to the drawing board....