Preheat oven to 350°F and line muffin pans with cupcake liners. Beat the butter and sugar on high speed in a stand mixer with a paddle attachment or using a handheld electric mixer for 3 minutes until creamy and light. Scrape the sides and bottom of the bowl using a rubber spatula.
Add egg whites and vanilla, beating on medium-high speed until combined. Beat in sour cream.
Next, whisk the flour, baking powder, baking soda, and salt together in a separate large bowl. Add to the batter, mixing on low speed just until combined, then scrape the bottom and sides of the bowl.
With the mixer running on low speed, add the milk, stirring just until combined. Do not overmix.
Add the sprinkles and stir to combine.
Fill each cupcake liner about ⅔ full with batter, then bake for 19-22 minutes, until a toothpick inserted in the center of a cupcake comes out clean.
Remove the cupcakes from the oven and transfer them onto a wire rack to cool completely before frosting.
Add any frosting and decorations to the cupcakes that you desire, and serve.