Mix all ingredients except milk with a hand mixer until well combined and fluffy. Add milk as needed to reach desired consistency.
Frosting can be tightly covered and set aside in the fridge for up to 48 hours before using, just give it a good whip with your hand mixer to re-blend it and soften it up before using.
Preheat oven to 350 F. Grease, or line 2 cupcake tins with cupcake papers or parchment.
Combine the flour, cornstarch, salt and baking powder in a medium bowl and set aside.
In the bowl of stand mixer cream together the butter and sugar until fully incorporated and light and fluffy. Add the egg whites one at a time (save the yolks to make pudding later!), beating well after each addition. Add the oil and vanilla and mix until combined.
Add 1/3 of the flour and combine, then add 1/4 cup of milk, mixing until just combined. Repeat this process twice more, so that all the flour and milk is stirred in. Do not over mix.
Remove the bowl from the mixer and stir in the sprinkles by hand.
Evenly distribute the batter between the 24 cupcake cups. Bake for 20 - 25 minutes, or until a toothpick inserted in the center comes out clean. Allow to rest until completely cooled (or overnight) before frosting.