Preheat oven to 350°F. Line a cupcake tin with 12 liners; set aside.
In a medium size bowl, whisk together flour, baking powder, baking soda, and salt; set aside.
In a large bowl, whisk melted butter and sugar together, then whisk in sour cream, buttermilk, vanilla extract until combined. Slowly mix in dry ingredients until just combined; set aside.
In a medium size bowl with a hand mixer or whisk, or stand mixer fitted with a whisk attachment, beat egg whites until fluffy and thick, about three minutes. Fold whipped egg whites into the batter until no noticeable lumps of egg whites appear in the batter.
Divide batter among the 12 cupcake liners and bake for 20 minutes or until a toothpick inserted comes out clean. Let cupcakes cool on a wire rack.