Using a medium size sauce pot, bring an inch or so of water to a boil.

In a medium heat safe metal bowl that is large enough to rest on top of the sauce pot, whisk together the egg whites and brown sugar. (Use a kitchen aid metal mixing bowl and whisk attachment to save a dish later.)

Remove bowl from heat, and use an electric mixer to whisk the mixture until a stiff glossy meringue forms.

Once the meringue cools down to roughly room temperature, cut the butter into several pieces and add it into the bowl one piece at a time.

Beat for another few minutes until the buttercream is light and fluffy, and the butter flavor becomes less prominent. (Buttercream may look curdled at first but will come together when mixed long enough)

Add in the vanilla extract, caramel sauce, and fleur de sel, and whip until thoroughly combined.

To lessen the air bubbles in the buttercream, switch to a paddle attachment on your mixer and mix on low for 5-10 minutes. To speed up the process, see notes for "tempering" your buttercream.