Place egg whites and powdered sugar in a stand mixer bowl. Attach the whisk and combine ingredients on low and then whip on high for 1 minute to dissolve the powdered sugar
Add in your salt and vanilla extract
Add in your butter in chunks and whip with the whisk attachment to combine. It will look curdled at first. This is normal. It will also look pretty yellow. Keep whipping.
If your buttercream looks curdled, remove about 1/3 cup of buttercream and melt it in the microwave for 10-15 seconds until JUST barely melted. Pour it back into the whipping buttercream to bring it all together.
(Optional) Add your drop of purple food coloring. Whip on high with the whisk attachment for 8-10 minutes until it's very white, light and shiny. Taste the buttercream, if it tastes like sweet ice cream then it's ready!
Switch to a paddle attachment and mix on low for 15-20 minutes to make the buttercream very smooth and remove air bubbles. This isn't required but if you want really creamy frosting, you don't want to skip it.