Why It Works
- Using toasted sugar brings the overall sweetness into balance, adding complexity of flavor.
- Liquid ingredients help the sugar dissolve and can add a note of flavor.
- At 155°F (68°C), the egg yolks will be fully cooked.
- Testing the finished buttercream with a thermometer helps rule out problems related to temperature, a common concern in recipes built on butter.
This silky-smooth frosting has a mellow sweetness and rich custard flavor that complements coconut cake as readily as it does chocolate. Like Swiss buttercream, it's easy to spread and pipe, so it makes cake-decorating a breeze. It can also be doctored with various extracts, spices, nut butters, or melted chocolate to taste.
- 5 1/2 ounces egg yolk (shy 2/3 cup; 155g), from about 10 large eggs
- 5 1/4 ounces plain or lightly toasted sugar (about 3/4 cup; 145g); see note
- 1 ounce bourbon, brandy, rum, tea, or coffee (about 2 tablespoons; 30g)
- 1/4 ounce vanilla extract (1 1/2 teaspoons; 7g)
- 1/2 teaspoon (2g) Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
- 16 ounces unsalted butter (4 sticks; 455g), softened to about 65°F (18°C)
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.
Place bowl over steaming water, stirring and scraping constantly with a flexible spatula, until egg yolk syrup reaches 155°F (68°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.
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°F (22°C).
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°F and re-whip before using.
Troubleshooting: If warmer than 74°F (23°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°F (20°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.
Though technically optional, using quick-toasted sugar will dramatically tame the sweetness of this buttercream, while also adding subtle depth of flavor. It's a lovely touch for basic vanilla, but not nearly as vital when stronger flavors, like chocolate or spices, come into play.