Preheat the oven to 350. Line 16 wells of a muffin tin with paper liners and set aside.
In a large bowl, whisk together the flour, sugar, baking soda, baking powder, and salt. In a medium bowl, mix together the butter, egg whites, vanilla, sour cream, and milk. Pour the wet ingredients into the dry ingredients, and mix until well combined.
Scoop the batter into the prepared muffin tin, filling the cups 2/3 full. Bake 18-20 minutes, until the tops spring back when touched lightly. Remove from the oven and cool completely.
To make the frosting, beat the butter until light and fluffy. Add the powdered sugar, vanilla, salt, and 1 tablespoon of milk, and mix. Add additional milk as needed to make a smooth frosting. Increase the speed to high and beat 3-5 minutes, until light and fluffy. Pipe onto the cooled cupcakes and decorate with sprinkles, if desired.