Preheat the oven to 350 degrees. Butter and lightly flour a 9 x 13 baking dish (or coat with baking spray).
Prepare the cake by whisking the egg whites, whole milk, and vanilla until well combined.
Sift the cake flour, sugar, baking powder, and salt together in a large mixing bowl.
Add the butter along with half of the egg white mixture and beat with a mixer.
Scrape the sides and bottom of the bowl, then slowly add the remaining egg white mixture and beat until just combined.
Fold in the sprinkles and pour into the prepared baking dish.
Place into the oven and bake for 30-35 minutes, or until a tester inserted into the center comes out clean.
Allow to cool on a wire rack completely.
Prepare the vanilla buttercream frosting by placing the room temperature butter in a large bowl.
Beat with a mixer on medium speed until smooth, creamy, and light in color.
Add the sifted powdered sugar and a pinch of salt then beat with the mixer until combined.
Add the vanilla and a few tablespoons of milk at a time until your frosting is at the desired consistency.
Finish the cake once it has cooled completely by coating the cooled cake evenly with the vanilla buttercream frosting.
Sprinkle the top with sprinkles then slice and serve. Enjoy.