• Wipe the bowl of an electric mixer with a 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 the temperature reaches 160°F, or until the sugar has completely dissolved and the egg whites are hot and foamy. Carefully wipe the bottom of the bowl dry.
• Attach the bowl to the mixer fitted with the whisk attachment and begin to whip, until the meringue is thick, glossy, and stiff peaks are achieved. The bottom of the bowl must no longer feel warm to the touch. This should take 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. This may take several minutes.
• If the buttercream curdles, simply keep mixing and it will come back together and be smooth. If the buttercream is too thin and runny, refrigerate for about 15 minutes before continuing mixing with the 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.

To make ahead:

• Keep in an airtight container in the refrigerator for up to one week or in the freezer for up to 2 months. Let come to room temperature and beat on low with the paddle attachment, until creamy, before using.

To use under fondant:

• Frost the cake as smoothly as possible. Place in the refrigerator until the buttercream has hardened before covering in fondant. Cover with fondant straight from the refrigerator.