Line a 12 cup muffin tin with paper cupcake liners.
Preheat the oven to 350 degrees F.
In a medium bowl, whisk together flour, baking soda, baking powder and salt.
In a mixer fitted with the paddle attachment, cream the butter and sugar together on medium speed until pale and fluffy, about 3-4 minutes, scraping down the sides as needed.
Add the egg whites and vanilla and mix on medium speed until well combined, about 3 minutes, scraping down sides as needed.
Add the sour cream and continue beating until incorporated. Reduce the mixer speed to low.
Add the dry ingredients in thirds, mixing only until just combined. Add the milk and continue mixing until just incorporated.
Fold in the sprinkles with a rubber spatula until evenly distributed, but do not over-mix as the color from the sprinkles will start to bleed into the matter.
Fill cupcakes lines about 2/3 full. You will have some batter left over to make an extra 2-3 cupcakes.
Bake in the preheated oven, about 20-25 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven and set cupcake tin on a wire rack to cool. When cupcakes are cool enough to handle, remove them from the tin to finish cooling on the rack.