Place egg whites and sugar into the bowl of a stand mixer, and whisk until combined.
Set mixer bowl over a saucepan filled with 1 inch of gently simmering water, making sure bottom of the bowl does not touch the water. Heat, whisking constantly, until sugar is fully dissolved, or a thermometer registers 160°F. (Rub the mixture between two fingers to test for grittiness.)
Dry off bottom of the bowl and place back on the stand mixer. Whisk on high speed until the meringue is glossy and can hold stiff peaks, and the bowl is cooled, usually about 5 to 10 minutes, but could be longer.
Switch from the whisk to the paddle attachment.
Slowly add the cubed butter piece by piece, beating well on medium speed after each addition. When all the butter has been added, scrape down the sides of the bowl and raise the speed to medium-high. Beat until the buttercream is thick and fluffy.
When buttercream is fluffy and smooth, add the salt, vanilla, and any other extracts or flavorings being used, and beat on medium-high until incorporated and buttercream is smooth.
Buttercream can be made ahead of time and frozen. To use, bring back to room temperature, then beat until smooth again.
