Preheat oven to 325°F (165°C). Line a 12-cup muffin tin with cupcake liners.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a small bowl, combine milk, sour cream, and vanilla.

In a mixing bowl, beat the butter and sugar until light and fluffy.

Add egg whites one at a time, mixing well.

Add half of the wet ingredients and half of the dry ingredients. Mix to combine. Add the remaining wet and dry ingredients and mix until just incorporated.

Scoop batter into liners (¾ full, about ¼ cup per cupcake). Bake for 20–22 minutes until tops spring back to touch.

Cool completely on a wire rack.

In a bowl, beat butter and powdered sugar until crumbly. Add vanilla and milk. Beat until smooth and fluffy.

Mix in a drop of red gel coloring until light pink. Adjust as needed. Place the buttercream in a piping bag fitted with a round or star tip. Pipe a swirl from the outside in, ending in the center.

Serve or chill until ready to enjoy.