Prepare 3 x 6" round cake pans by lining the bottom with parchment paper.
Lightly grease the pans and parchment and set aside.
Preheat oven to 350°F/177°C.
In a large bowl, sift together the cake flour and baking powder. Set aside.
In a stand mixer bowl fitted with a paddle attachment, cream the butter until lightened.
Add in the granulated sugar and continue to beat, until fluffy.
Combine the vanilla extract, egg + egg yolk in a small bowl.
Lower the mixer speed and while continuing to beat, add in the vanilla and egg in additions, until all of it is absorbed into the butter mixture.
Stop the mixer and give it a scrape with a spatula.
With the mixer on low speed, alternate adding in the cake flour and milk, starting and ending with the cake flour.
Stop the mixer and fold in the confetti sprinkles with a spatula.
Divide the cake batter equally into the prepared cake pans.
Even out the surface with an offset spatula.
Give the pans a tap to release any trapped air bubbles.
Bake the cakes at 350°F/177°C for 20-24 minutes, until a toothpick inserted comes out clean or with the slightest crumbs clinging to it.
Let the cakes cool on a wire rack.