Heat oven to 350 degrees F.
Whisk together flour, salt and baking powder in mixing bowl; set aside
In another mixing bowl (or bowl of stand mixer), beat butter for two minutes until fluffy. Add sugar; beat for another two minutes. Scrape down bowl with spatula.
Slowly add egg whites to sugar mixture while beating on medium speed until fluffy, about two minutes.
Add vanilla extract and scrape down bowl with spatula.
Slowly add flour mixture and milk, alternating by adding a little flour first, then half the milk, then flour, etc., Beat for a couple minutes until smooth and fluffy.
Fill cupcake liners to 2/3 full and bake for about 17-20 minutes. If you are making tall muffins, it takes a few extra minutes. Cupcakes are done when a toothpick comes out clean when inserted at the top, center.
Let cool completely.