Once the sugar has dissolved, transfer the mixing bowl to a stand mixer, if you used a separate bowl to whisk the egg white-sugar mixture. Use the whisk attachment and beat on high for 5 to 10 minutes. The egg whites will turn into a bright white, thick, and glossy meringue that forms stiff peaks. You can use an electric beater too, but it will take more time.
06Once the stiff peak stage is reached, swap the whisk attachment for the paddle attachment. The paddle attachment creams the butter better. Also, make sure the meringue is at room temperature and not hot or else it will melt the butter.
07Add the butter, a tablespoon at a time, at medium speed. Add the next tablespoon, once the previous tablespoon of butter is fully incorporated.
08When all the butter has been added the mixture will be soft, almost liquidy, and might even split, but do not panic! Crank up the speed to high, and beat for another 2 to 3 minutes, and then, almost magically, the butter and egg whites will come together to form a silky-smooth frosting.
09Add the vanilla extract and salt and beat well to combine.
10If you need to color this frosting, use food gel.