In a medium bowl, whisk or sift together flour, baking powder, and salt. Set aside.
In a mixer fitted with the paddle attachment, beat together butter and sugar on medium speed until light and fluffy, about 3 minutes. Scrape down the sides and bottom of the bowl as necessary. On medium speed, beat in egg whites, one at a time, beating well after each addition. Add vanilla extract and beat until combined. With the mixer on low speed, add half of the dry ingredients and beat just until combined. Add milk and sour cream/heavy cream and beat until combined. Add the other half of the dry ingredients and beat slowly just until combined. Stir in sprinkles. Do not over mix – the less you mix, the lighter the cupcakes will be.
Divide batter evenly between the cups, filling them about 3/4 full. Bake for 15-20 minutes until a toothpick inserted into the center comes out clean. Let cupcakes cool in the pan for 10 minutes, then remove from pan and allow to cool completely on a wire rack. Frost cupcakes with frosting and top with sprinkles.
Unfrosted cupcakes can be kept tightly covered in the fridge for up to 3 days, or in the freezer for up to 2 months. Thaw, still covered, on the counter or overnight in the fridge (same for the frosting but bring it to room temperature and stir well before using to frost the cupcakes). Frosted cupcakes can be kept at room temperature for 24 hours or in the fridge for up to 3 days. Bring to room temperature before serving (at least 1 hour).