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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.