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.
½ cup salted butter,, 1 cup granulated sugar
Add egg whites and vanilla, beating on medium-high speed until combined. Beat in sour cream.
3 large egg whites,, 2 teaspoons clear vanilla extract, ½ cup 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.
1¾ cups cake flour, 1 teaspoon baking powder, ¼ teaspoon baking soda, ¼ teaspoon salt
With the mixer running on low speed, add the milk, stirring just until combined. Do not overmix.
½ cup milk,
Add the sprinkles and stir to combine.
½ cup rainbow sprinkles
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.