In a small saucepan over medium heat, add 10 tablespoons (5oz/142g) of the granulated sugar and the water. Bring to a boil without stirring.

Boil the mixture for 5-7 minutes, until it turns a dark amber color and begins to smell a little smoky.

Immediately remove the pan from the heat and carefully add the heavy cream to stop the cooking. (Note: It will bubble up.) Stir until smooth and any hardened sugar dissolves. Transfer to a small bowl, then stir in the vanilla and salt. Set aside to cool.

Bring a double boiler to a simmer. Place the egg whites and the remaining 10 tablespoons (5oz/142g) of granulated sugar in the double boiler and whisk for 4-5 minutes, until the sugar is dissolved and the mixture no longer feels gritty when a small amount is rubbed between your fingers.

Transfer the egg white mixture to the bowl of a stand mixer fitted with a whisk attachment (or use a handheld electric mixer). Whip on medium speed for about 10 minutes, until the mixture holds stiff, glossy peaks and has cooled completely to room temperature.

Switch to a paddle attachment. With the mixer on medium speed, gradually add the butter, a few pieces at a time, until fully incorporated and fluffy.

Beat in the caramel until just combined. Use the frosting straight away or store in an airtight container at room temperature for up to 2 days, in the refrigerator for 2 weeks, or in the freezer for 2 months. Bring up to room temperature on the counter for a few hours and re-beat before using.