Adapted from Flour Child
It’s very important that your caramel be no warmer than room temperature when you add it to the frosting, or your frosting will melt.
6 large egg whites
Place a saucepan with about 2 inches of water over medium-low heat and bring to a simmer. Combine egg whites, sugar and salt in the metal bowl of a stand mixer. Place bowl over pan of simmering water and whisk constantly until mixture an instant-read thermometer reads 160°F. (Sugar will be dissolved and mixture will start to resemble marshmallow fluff.)
Remove bowl from heat and attach to stand mixer. Beat with the whisk attachment until stiff meringue peaks form and the outside of the bowl no longer feels warm, about 10 minutes. Switch to paddle attachment. On low speed, add one cube of butter at a time, waiting until incorporated before adding the next. Once all butter is added, on low speed slowly pour in caramel. Add vanilla and continue beating until smooth. (If frosting curdles, just keep beating! If frosting is soupy, place mixer bowl in fridge for 10-15 minutes and beat until smooth. Repeat if necessary.)