Preheat your oven to 350°F. Line two muffin pans with 16 paper cupcake liners.
Sift the flour, baking powder, and salt into a medium bowl.
In another medium bowl, whisk together the melted butter, sugar, vanilla, and egg whites until light and fluffy. Combine the milk and sour cream.
Alternate adding the dry ingredients and the milk and sour cream mixture, using a silicone spatula to fold in each addition, beginning and ending with the dry ingredients. Do not over-mix. Finally, fold in the sprinkles.
Scoop the batter into the prepared muffin pans. I use a (¼ cup/ 60 ml) scoop to ensure evenly sized cupcakes. Place the muffin pan on a cookie sheet (to prevent the bottoms of the cupcakes from browning too much), and bake until lightly golden and a knife inserted in the centre of a cupcake comes out clean, 15–18 minutes.
Prepare the cream cheese icing.
When the cupcakes are completely cool, decorate them with cream cheese icing and more sprinkles.