French Buttercream Frosting

Ingredients

• 5 1/2 ounces egg yolk , from about 10 large eggs
• 5 1/4 ounces plain or lightly toasted sugar ; see note
• 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. Fill a wide pot with at least 1 1/2 inches water, with a thick ring of crumpled aluminum foil placed on the bottom to act as a "booster seat" that will prevent the bowl from touching the bottom of the pot. Place over high heat until steaming-hot, then adjust temperature to maintain a gentle simmer. Combine egg yolks, sugar, bourbon or other liquid ingredient, vanilla, and salt in the bowl of a stand mixer.
2. Place bowl over steaming water, stirring and scraping constantly with a flexible spatula, until egg yolk syrup reaches 155 degrees F (68 degrees C). This should take only about 5 minutes; if the process seems to be moving slowly, simply turn up the heat. Once it's ready, transfer to a stand mixer fitted with a whisk attachment and whip at high speed until mixture is fluffy, stiff, and beginning to ball up around the whisk, about 8 minutes.
3. With mixer still running, add butter 1 or 2 tablespoons at a time, waiting only a second or two between additions. In the end, the buttercream should be thick, creamy, and soft but not runny, around 72 degrees F (22 degrees C).
4. Use buttercream right away, or transfer to a large zipper-lock bag, press out the air, and seal. Buttercream can be refrigerated for up to 2 weeks and frozen for up to several months. (The main issue with longer storage in the freezer is odor absorption, not spoilage.) Rewarm to 72 degrees F and re-whip before using.
5. Troubleshooting: If warmer than 74 degrees F (23 degrees C), the buttercream will be soft and loose; pop it in the fridge for 15 minutes and re-whip to help it thicken and cool. If colder than 68 degrees F (20 degrees C), the buttercream will be firm and dense, making it difficult to spread over cakes and slow to melt on the tongue, creating a greasy mouthfeel. To warm, briefly set over a pan of steaming water, just until you see the edges melting slightly, then re-whip to help it soften and warm. French buttercream can be fixed according to the same rules for Swiss buttercream. For more information, see the troubleshooting guide and video here.