Preheat the oven to 350°F.
Line a 12-cup muffin tin with paper liners and set aside.
To make the cake, whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl.
In a medium bowl, mix butter, egg whites, sour cream, milk, and vanilla with a hand mixer. Pour wet ingredients into dry ingredients and mix until just combined, but do not overmix.
Scoop the batter into the prepared muffin tin, filling the cups two-thirds full.
Bake for 18–20 minutes, until the tops spring back when touched lightly. Remove from the oven and cool completely.
While cupcakes cool, make the frosting: Beat butter with a hand mixer on low speed until light and fluffy. Add powdered sugar, vanilla, salt, and 1 tablespoon 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.