Make Cupcakes
Preheat oven to 350 degrees F.
Whisk together flour, sugar, salt, baking soda, and baking powder.
Whisk together melted butter, egg whites, vanilla, sour cream, and warm milk.
Combine wet and dry ingredients.
Fill cupcake papers ⅔ full.
Bake 18 minutes.
Make Buttercream
Cream butter.
Add confectioners sugar in two batches.
Add salt, 1 tbsp cream, 1 tsp vanilla, and 1 tsp milk. Mix until fluffy.
Assemble
Transfer buttercream to a piping bag.
Pipe buttercream onto cupcakes.
