Measure egg whites in your mixing bowl.
Tare. Measure powdered sugar in bowl. (Or in separate bowl, either way, add powdered sugar at this time)
Using the paddle attachment, beat on medium/high (6 on KA) for 5 minutes. You are essentially creating a soft peak, glossy royal icing at this point. If it does not "peak", do not worry, our main concern is to dissolve the powdered sugar. You can incorporate more air into it down the line.
Make sure your butter is SOFT. Room temperature often isn't soft enough for me, so I'll put it in the microwave for 10 more seconds to make sure it's super soft to the touch. This will allow air to beat into the mixture easier.
Set mixer to medium, gradually add butter. Scrape bowl.
Don't forget to add your vanilla! Now, beat for 5-10 minutes on medium/high (6 on KA) until light and fluffy. To help remove larger air bubbles that may have formed, I reduce it to a low setting and let it mix for approx 2 minutes.
Fluffy delicious buttercream! Nom nom nom. See below for storage and other notes!