Preheat oven to 350°F (177°C) and line cupcake pans.
Whisk flour, baking powder, baking soda, and salt.
In another bowl, mix melted butter and sugar.
Add egg whites, sour cream, milk, and vanilla.
Combine wet and dry ingredients until smooth.
Fill liners 2/3 full and bake 20–22 minutes.
Cool completely before filling.
Fill the Cupcakes
Cut a small center hole in each cupcake.
Fill with strawberry filling and replace top piece.