Place the egg whites and sugar in the mixing bowl of a stand mixer. Set the bowl on top of a small pot of simmering water. Whisk the mixture until the sugar melts and the mixture become hot to touch and very thin. Remove from the heat and attach to the stand mixer. Whisk on a high speed until stiff peaks form. Add butter one tbsp at a time. Add salt, and extract. Keep mixing until the buttercream becomes smooth. Place in a piping bag and pipe between macaron cookies.