Preheat the oven to 350°F. Line 14 cavities on a muffin pan with cupcake liners. I spread them ~evenly between two 12-count muffin pans.
Add ½ cup room temperature butter and 1 cup sugar to a large bowl (the bowl if you stand mixer, if using). Beat together using a handheld mixer or stand mixer with the paddle attachment, for approx. 2 minutes on high speed. Mixture should be smooth and creamy, and light in color.
Use a spatula to scrape the sides and bottom of the bowl as needed. Add in 3 large egg whites and 2 tsp vanilla extract. Beat on medium-high speed until combined, then beat in ½ cup room temperature sour cream. Make sure to scrape the sides so everything is evenly incorporated.
Set the mixer on low speed and add 1¾ cup sifted cake flour, ¾ baking powder, ¼ tsp baking soda, and ¼ tsp salt. Mix until just incorporated. While the mixture is running on low speed, slowly pour the milk in until combined. Do not over-mix
Add in ½ cup sprinkles and gently fold in the sprinkles. The batter with be on the thicker side.
Scoop the batter into the muffin tin liners. Fill ⅔ full, I found this to be roughly 5 Tbsp, or 2½ scoops of my cookie scoop.
Bake for 18-22 minutes, or until a toothpick in the center of a cupcake comes out clean. (For 30 mini cupcakes, bake 10-12 minutes.) Allow cupcakes to cool completely before frosting.
Leftover cupcakes keep well covered tightly in the refrigerator for 3 days.