Preheat the oven to 350 degrees F. Spray organic oil or spread butter onto 3 4-inch diameter cake pans. Line each cake pan with a circular piece of parchment paper.
Place the all-purpose flour, cake flour, baking soda, baking powder, and kosher salt into a medium sized bowl. Whisk and set aside.
Add unsalted butter and granulated sugar in a bowl of an electric mixer. Mix the ingredients on medium speed until the mixture is while incorporated and fluffy. (Roughly 1.5 minutes)
Place the egg, sour cream, milk, and vanilla extract in a measuring cup. Whisk together and set aside.
Stir in half of the flour mixture and half of the liquid mixture. Once fully incorporated. Turn off the mixer and add the remaining flour mixture and liquid mixture.
Beat on medium speed into the batter is well mixed.
Add the sprinkles. Once added, stir the cake batter by hand with a spatula.
Evenly distribute the batter between the prepared cake pans. Bake for 18 to 25 minutes, or until the inserted toothpick comes out clean from the center of the cake pan.
Cool the mini funfetti cake completely, before frosting.