This is Rose Levy Beranbaum's recipe from her book "The Cake Bible"
It is much less sweet than most buttercream recipes
Yield: 4 cups
6 large egg yolks
3/4 cup sugar
1/2 liquid cup corn syrup
2 cups unsalted butter, softened
optional: 2 - 4 tbsp. liqueur
Note: You can use 1 tsp. vanilla extract in place of the liqueur
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 elecctric hand-helf mixer, beat the syrup into the yolks in a steady stream. Don't allow the 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. 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 option flavoring. Place in an airtight bowl. Bring to room temperature before using. Rebeat to restore texture.