Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.
Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins)
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment (optional). Slowly add cubed butter and mix until smooth.
Add flavorings as desired whip until smooth.
Notes
This recipe makes enough to frost a two layer 8" cake or a three layer 6" cake.
The recipe can be made with 3 cups of butter instead, if you need more frosting.