- In the bowl of an electric mixer, add the egg whites and sugar.  Whisk briefly by hand until combined. 
- Fill a saucepan with a few inches of water and bring to a simmer over medium heat.  Place the mixer bowl on top of the saucepan to create a double-boiler.  Stirring intermittently, heat the egg mixture until it reaches 150 to 160 on a candy thermometer. 
- Once hot, carefully return the mixer bowl to the stand mixer.  Fitted with the whisk attachment, whip the egg white mixture on high until stiff peaks, about 8 minutes.  When done, the outside of the mixer bowl should return to room temperature. 
- Stop the mixer and swap the paddle for the whisk.  With the mixer on low, add in the vanilla and butter, a couple tablespoons at a time.  Once all of the butter has been added, turn the mixer up to medium-high and mix until smooth.  If the mixture looks curdled, just keep mixing until it is smooth (this could take up to about 5 minutes).  If it appears soupy, place the mixer bowl in the fridge for 10 to 15 minutes, then mix until smooth.  Tint with gel food coloring, if desired. 
- Recipe may be doubled to frost and decorate a larger cake.