Over a double boiler, heat the whites and granulated sugar and salt, whisking constantly, until all the sugar is dissolved. Heat this slowly!! You don’t want scrambled eggs!
Place the mixture in a stand mixer and whip on high until a peak forms and the mixture is cool, about 5 minutes.
Turn the mixer to medium speed and add the softened butter a little at a time.
Mix in the mascarpone.
Add the powdered sugar, rum, espresso powder, and vanilla and mix until the texture is smooth, silky, and pipable.
Frost cupcakes and top with a dusting of cocoa powder.
Notes
Adapted from Joshua Russell's modern buttercream recipe.