Set up a double boiler by bringing about an inch of water to a bare simmer in a medium saucepan. The bowl of your stand mixer should nest inside without the water touching the bottom.

Whisk the egg whites and granulated sugar together in the stand mixer bowl. Place the bowl over the simmering water and whisk constantly until the mixture reaches 130°F on an instant-read thermometer and feels completely smooth when rubbed between your fingers, with no trace of graininess.

Transfer the bowl to the stand mixer and fit with the whisk attachment. Beat on high speed until the meringue reaches stiff, glossy peaks and the outside of the bowl feels cool to the touch.

Switch to the paddle attachment. Add all the butter at once and beat on medium speed until the butter is fully incorporated and a smooth, silky buttercream forms. If a few small butter pieces remain stubborn, briefly warm the outside of the bowl with a kitchen torch, then continue mixing.

If the buttercream turns soupy, continue mixing until all butter is incorporated, then refrigerate the bowl for 20 to 30 minutes and beat again until it comes together.

Add the salted caramel sauce and vanilla extract. Switch back to the whisk attachment and beat on medium-high until the buttercream is light, fluffy, and fully combined.