Ingredients
1 1/4 cups sugar
1/3 cup heavy cream
6 large egg whites
Pinch of kosher salt
1 pound (4 sticks) unsalted butter, room temperature, cut into tablespoon-sized pieces
1 teaspoon pure vanilla extract
Preparation
Make caramel: Pour 1/4 cup water into a medium heavy-bottom saucepan. Add 1/2 cup plus 2 tablespoons sugar. Cook over medium-high heat, stirring occasionally, until sugar is dissolved and syrup is clear. Continue to cook, without stirring, until syrup comes to a boil, washing down sides of pan with a wet pastry brush 2 or 3 times to prevent crystals from forming. Let syrup boil, gently swirling pan occasionally, until syrup is dark amber.
Immediately remove from heat; carefully pour in cream (caramel will bubble vigorously). Stir caramel until smooth. Transfer to a bowl; let cool.
Whisk together remaining 1/2 cup plus 2 tablespoons sugar, the egg whites, and salt in the heatproof bowl of a standing mixer. Set bowl over a pan of simmering water; whisk until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer.
Return bowl to mixer; fit mixer with whisk attachment. Beat on medium-high speed until stiff. Reduce speed to medium low; add butter, a few tablespoons at a time. Continue whisking until smooth.
Switch to paddle attachment. Add caramel and vanilla; beat on lowest speed until combined, 3 to 5 minutes. The buttercream can be made 2 days ahead and stored in the refrigerator. Before using, bring frosting to room temperature; return to mixer and beat with the paddle attachment until smooth.
