Preheat the oven to 350°F and line a cupcake pan with paper liners. Sift the flour, granulated sugar, baking powder, baking soda, and salt into a large mixing bowl and whisk to combine. In a separate bowl, combine the egg whites, vanilla extract, sour cream, melted butter, and warm milk; whisk until blended—lumps are expected at this stage. Pour the wet mixture into the dry ingredients and mix until smooth. Divide the batter evenly among the cupcake liners, filling each approximately two-thirds full. Bake for around 18 minutes, or until the tops spring back when lightly pressed.

For the buttercream, place the softened butter in the bowl of a stand mixer fitted with a paddle attachment and beat until creamy. Add the confectioners’ sugar in two additions, followed by the salt, milk, heavy cream, and vanilla extract. Continue to mix until the frosting is light and fluffy. Spoon the buttercream into a piping bag and apply a generous swirl to each cooled cupcake.