Preheat the oven to 350 F, and position a rack in the center of the oven. Line your muffin pans with 18 paper cupcake liners.
In the bowl of your stand mixer fitted with the paddle attachment, beat the softened butter with the sugar for about 3-5 minutes on medium speed, scraping the bowl frequently, until light and fluffy.
Beat in the egg whites, one at a time, until well combined.
In a separate bowl, sift together the flour, baking powder and salt. Set aside 1 tablespoon of the dry ingredients.
In another bowl or liquid measuring cup, whisk together the milk, sour cream and vanilla extract.
With the mixer on low, add the flour mixture in three additions, alternating with the milk mixture, starting and ending with the flour. Once you've added everything, mix well for about 15-30 seconds until the batter is well combined.
Toss the blueberries with the reserved 1 tablespoon of dry ingredients, then fold the blueberries into the cake batter.
Divide the batter between 18 cups, filling them about 3/4 full.
Bake the cupcakes for about 16-18 minutes, just until the tops spring back when gently touched. Cool in the pan for several minutes, then gently transfer the cupcakes to a cooling rack to cool completely before frosting. The tops of the cupcakes may settle and flatten a bit as they cool - this is totally normal.