Use the leftover caramel from this rich, sweet-and-salty frosting to give your cake or cupcakes (like our One-Bowl Chocolate Cupcakes) an extra-special swirl effect.
Combine 2 1/2 cups sugar and 3/4 cup water in a medium saucepan. Bring to a boil over high, stirring just until sugar dissolves. Cook, swirling the pot occasionally (not stirring), until dark amber, about 10 minutes. Remove from heat; slowly stir in cream until caramel is smooth (mixture will splatter). Set aside to cool.
Whisk together egg whites, kosher salt, and remaining 1 cup sugar in the bowl of a stand mixer set over (not in) a pot of simmering water. Whisk until sugar is dissolved and mixture is smooth and warm to the touch. Transfer bowl to mixer fitted with the whisk attachment; beat on medium-high speed until mixture is fluffy and cool, and stiff peaks form, about 10 minutes. Add butter 2 tablespoons at a time, whisking well after each addition. Whisk in vanilla. Slowly pour in 1 1/2 cups caramel with mixer on medium speed, and whisk until incorporated. Increase to high speed (the mixture will look curdled), and continue to whisk until smooth, about 3 minutes. Switch to the paddle attachment and beat on low speed for 2 minutes to remove air bubbles. Frost cake or cupcakes. Drizzle remaining caramel over the top, then swirl with a skewer or toothpick. Sprinkle with flaky salt, if desired.
Buttercream can be stored in an airtight container in the refrigerator up to 3 days, or in the freezer up to 1 month. Bring to room temperature before using, and beat until smooth.