French Buttercream Frosting

Ingredients

• 5 1/2 ounces egg yolk, from about 10 large eggs
• 5 1/4 ounces plain or lightly toasted sugar
• 1 ounce bourbon, brandy, rum, tea, or coffee
• 1/4 ounce vanilla extract
• 1/2 teaspoon Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
• 16 ounces unsalted butter, softened to about 65°F

Directions

1. Set a wide pot on the stove with at least 1 1/2 inches water in it, and nestle a sturdy coil of crumpled aluminum foil in the bottom so the mixing bowl can sit above the base of the pot. Heat the water over high heat until it is very steamy, then lower the heat enough to hold it at a gentle simmer. In the bowl of a stand mixer, stir together the egg yolks, sugar, bourbon or other liquid ingredient, vanilla, and salt.
2. Set the bowl over the steaming water and cook the mixture, stirring and scraping the bowl the whole time with a flexible spatula, until the egg yolk syrup registers 155 degrees F (68 degrees C). It should take about 5 minutes, so if it is creeping along, nudge the heat higher. When it is ready, move the bowl to a stand mixer fitted with the whisk attachment and beat on high speed until the mixture is thick, fluffy, firm, and starting to gather around the whisk, about 8 minutes.
3. While the mixer is running, add the butter 1 or 2 tablespoons at a time, pausing only briefly between each addition. When all the butter is in, the buttercream should look thick, creamy, and soft without being runny, and it should be around 72 degrees F (22 degrees C).
4. Use the buttercream immediately, or spoon it into a large zipper-lock bag, squeeze out the air, and seal it. It can be refrigerated for up to 2 weeks or frozen for up to several months. The main concern with a longer stay in the freezer is picking up odors, not spoilage. Before using, bring it back to 72 degrees F and whip it again.
5. For troubleshooting, check the temperature first. If the buttercream is warmer than 74 degrees F (23 degrees C), it will be too soft and loose; chill it for 15 minutes, then whip it again so it cools and thickens. If it is colder than 68 degrees F (20 degrees C), it will turn dense and firm, which makes it harder to spread on cakes and slower to melt, giving it a greasy feel. To warm it, place it briefly over a pan of steaming water just until the edges begin to melt a little, then whip again to soften and warm it. French buttercream can be repaired using the same general approach as Swiss buttercream.