Wipe the bowl of an electric mixer with paper towel and lemon juice or vinegar to remove any trace of grease. Make a double boiler by placing the mixer bowl over a saucepan of simmering water, making sure the bowl doesn’t touch the water.
Add the egg whites and sugar to the bowl, whisking constantly but gently, until temperature reaches 140°F, or until the sugar has completely dissolved and the egg whites are hot.
Attach the bowl to the mixer fitted with the whisk attachment and begin to whip until the meringue is thick, glossy, and the bottom of the bowl no longer feels warm, about 7 to 10 minutes.
Switch over to paddle attachment and, with mixer on low speed, add the butter cubes, one at a time, until incorporated. Continue beating until it has reached a silky smooth texture. If the buttercream curdles simply keep mixing and it will come back to smooth. If the buttercream is too thin and runny, refrigerate for about 15 minutes before continuing mixing with paddle attachment until it comes together. Add the vanilla and salt, continuing to beat on low speed until well combined.
Add additional flavors, purees, or mix-ins as desired.
Keep in airtight container in refrigerator for up to one week or in the freezer for up to 2 months. Let come to room temperature and rewhip in the mixer with the paddle attachment before using.
Frost the cake as smooth as possible. Place in refrigerator until the buttercream has hardened before covering in fondant. Cover with fondant straight from the refrigerator.