Mix the egg whites, powdered sugar and salt in a bowl. On low speed with a paddle attachment, mix everything until there are no dry patches.
Stop mixing and use a spatula to scrape down the sides and base of the bowl.
Keep mixing on medium speed for about 5 minutes.
Turn the mixer to low and begin incorporating the room temperature butter. Be sure to incorporate only 2 tablespoons at a time.
Once the butter is fully incorporated, add your vanilla extract.
Stop mixing, scrape down the sides and bottom and turn the speed back up to medium. Allow the buttercream to beat for 10 minutes.
Use right away to decorate or store in the fridge for later use (will stay good for about 2 weeks).