Meanwhile, make the frosting: Beat the confectioners' sugar and egg whites in a stand mixer fitted with the paddle attachment on medium-high speed for 1 to 2 minutes. Slowly add in the butter, a tablespoon or two at a time, and continue mixing. Once combined, increase the speed to high and beat until smooth. If the frosting looks curdled and separated, remove one-quarter of it to a microwave-safe bowl and melt in the microwave for about 10 seconds, then pour back into the rest of the frosting and whip on high. This should bring the frosting together; if not, repeat this process again. Stir in the vanilla and salt.