Preheat oven to 350 degrees (F). Line a 12 mold muffin tin with paper 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 combo and beat on low until *just combined. Over-mixing will result in a dryer/denser cake.
Divide the cupcake batter evenly into the 12 molds, 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. Add additional confectioners sugar as needed, stopping when you've reached your desired consistency. Place frosting in piping bag, or simply slather onto cooled cupcakes.
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