Step 1In the bowl of an electric mixer, mix the egg yolks until thick and pale lemon-colored.
Step 2Put the sugar, one-fourth cup water and corn syrup in a medium, heavy-bottomed saucepan. Bring to a simmer over medium-high heat and cook, wiping down the sides at the beginning, until the syrup registers 215 degrees on a candy thermometer, about 5 minutes. Remove from the heat.
Step 3With the mixer on low, carefully pour the hot syrup into the eggs in a slow stream. Once the syrup has been poured in, turn the mixer to high speed and whip until the outside of the bowl is room temperature to the touch, about 15 minutes.
Step 4Add the cubes of butter, a few at a time. Whip until all the butter is incorporated.
Step 5Add the chocolate or flavorings, if using. The butter cream should be stiff and glossy and spreadable. If it's too warm and is too liquidy, just put the mixing bowl of butter cream (and the whip too) into the refrigerator for 15 minutes or so until it firms up. Then whip it at high speed until it's spreadable. The butter cream can be stored in the refrigerator if not using, but bring it back to room temperature and re-whip it at high speed to get it back to the proper consistency before frosting.
Add 4 ounces melted bittersweet chocolate or 1 1/2 tablespoons coffee extract or other extracts or liqueurs (about 1 to 2 teaspoons).