Combine egg whites and sugar in a heat proof bowl over a pan of simmering hot water.
Whisk continually until mixture becomes warm to touch and sugar has dissolved.
Pour mixture into bowl of stand mixer and whisk on high until stiff peaks form and the mixture has cooled (just feel the bowl - it should no longer feel warm to the touch).
Once the mixture has cooled (and not before!), switch to a paddle attachment and start adding the butter piece by piece with the mixer on med-high speed (don't worry if the mixture looks soupy or curdled - it will all come together with enough mixing). Once all the butter is added, keep beating until the mixture becomes silky and shiny.
Stop the mixer, scrape down the sides, and add the vanilla and 1/2 cup salted caramel (from one of the two portions of caramel). Beat on medium speed until combined. Taste the mixture and add more salted caramel if you prefer.
Refrigerate the buttercream if you want the mixture to be a little firmer before spreading over the cake, or if you have made it ahead of time.