1. Combine egg whites, sugar, and salt in the bowl of a stand mixer set over (not in) a pot of simmering water. Whisk constantly until sugar has dissolved, and mixture is warm and feels completely smooth when rubbed between fingertips. Transfer bowl to mixer fitted with the whisk attachment. Starting on low speed and gradually increasing to medium-high, beat until stiff, glossy peaks form, about 2 minutes. Continue beating until bottom of bowl is cool to the touch, about 10 minutes.

2. Reduce speed to medium-low and beat in coconut oil, a few tablespoons at a time, then butter, a few pieces a time, until combined, scraping down bowl as needed. Switch to paddle attachment and beat on low until smooth, about 1 minute more. Buttercream can be refrigerated in an airtight container up to 1 week; return to room temperature and beat until smooth before using.