Rum and Vanilla Bean Buttercream
In the bowl of an electric mixer fitted with the paddle attachment, beat butter until fluffy and pale. Set aside.Advertisement
Place sugar in a shallow bowl. Scrape in vanilla seeds; grind together with a wooden spoon. Sift to remove vanilla-bean residue. In clean bowl of mixer, set over simmering water, whisk flavored sugar, salt, and egg whites until warm and sugar has dissolved, 2 to 3 minutes. With whisk attachment, beat mixture, starting on low speed and gradually increasing to high, until fluffy and cooled, 12 to 15 minutes.
Switch to paddle attachment; add butter a few tablespoons at a time, incorporating fully after each, on low speed until completely smooth. Add vanilla extract and rum; combine. Use immediately, or refrigerate, covered, up to 3 days or freeze up to 2 weeks. Bring to room temperature before using, and beat on lowest speed with paddle until smooth.