Preheat the oven to 350°. Grease two 8-inch round cake pans and set aside.
In the bowl of a stand mixer whisk together the flour, sugar, baking powder, baking soda and salt..
Add the buttermilk, milk, vanilla, and oil to the bowl. Use the paddle attachment to mix on medium low until just barely fully combined. Using a rubber spatula, scrape down the sides and bottom of the bowl.
In a medium bowl whip the egg whites until medium stiff peaks form. Gently fold the egg whites into the cake batter until just a few streaks of egg whites remain. Add the sprinkles and fold into the batter until no streaks of egg whites remain and the sprinkles are evenly distributed.
Separate the batter evenly into the two prepared cake pans. Bake for 30-35 minutes, until a toothpick comes out clean from the center or the center of the cake springs back when gently pressed.
Let the cakes cool in the pan for 5 minutes before inverting upside down on a cooling rack. This will help the tops of the cakes flatten so they are easier to frost.
This step is optional but I like to make my cake rounds a day ahead so I can wrap them in plastic wrap and chill them in the fridge overnight. Frosting cold cakes is much easier than frosting freshly baked cakes. You can also store them in the freezer and frost them while frozen.
In the bowl of a stand mixer, use the paddle attachment to beat the butter until smooth and creamy.
Add in the powdered sugar, salt and vanilla. Mix on low until the powdered sugar is sufficiently absorbed by the butter and wont make a mess when the speed is increased. Increase the speed to medium high and beat for 2 minutes. Scrape down the sides and bottom of the bowl and add the milk. Mix on medium until smooth.
You may tint the frosting your chosen color or leave it white.