In a mixing bowl combine the egg whites, oil, milk, sour cream, and vanilla gently with a whisk. Then sift in your cake mix and sprinkles and gently stir.
Fill 24 cupcake liners ¾ full with cupcake batter and bake at 350℉ for 15-20 minutes (or until a toothpick stuck in the middle of a cupcake comes out clean).
Beat butter and vanilla together until fluffy. Add strawberry jam and beat until incorporated.
Add powdered sugar, 1 cup at a time. Mix until frosting is fluffy. It will seem kind of dry, so add 1-2 Tablespoons of milk until it's the consistency you like.
Pipe onto cooled cupcakes and top with sprinkles (if you want).
Notes
By adding the powdered sugar a little at a time you avoid a cloud of powdered sugar once you turn on the beaters.You can also jog (quickly turning the mixer on and off multiple times) the beaters a few times to also avoid getting sprayed with powdered sugar.Continue to mix the frosting until all of the powdered sugar has been added and is buttery soft and fluffy. The mix will seem to be a little dry. Add in 1-2 Tablespoons of milk at a time until the desired consistency is reached.Lastly, fill either a piping bag or a ziploc bag (or any resealable bag) with the corner cut off, and pipe the strawberry buttercream frosting onto the cooled confetti cupcakes and top them off with sprinkles.