Preheat oven to 350 degrees F. Line a muffin pan with cupcake liners; set aside.
In a medium bowl combine flour, baking powder, and salt; mix well to combine. Set aside.
In a large measuring cup or bowl, combine sour cream, milk, and egg whites; mix well to combine. Set aside.
In a large bowl or stand mixer fitted with the paddle attachment, beat melted butter and sugar on medium speed until well combined; about 2 minutes. Add lemon juice and lemon zest and beat for another minute on medium speed. Slowly add the flour mixture to the butter/sugar combination, beating until just combined. Add in sour cream/milk/egg combination and beat on low until just combined. Over-mixing will result in a dry/denser cake.
Divide the cupcake batter evenly in liners, and then place the pan in the oven. Bake for 18-20 minutes or until a toothpick inserted into the center of a cupcake comes out clean. Allow cupcakes to cool completely before topping with frosting.
To make the lemon buttercream, beat butter until light and fluffy. Slowly add 4 cups of sugar, beating on low until well combined. Add in lemon juice, zest and cream; beat until combined. Using a piping bag, frost cupcakes. Optional: Garnish with additional lemon zest.
Recipe by The Thrifty Couple at https://thethriftycouple.com/homemade-lemon-cupcakes-with-lemon-buttercream-frosting-recipe-gluten-free-version-too/