Preheat the oven to 350 degrees F. Line a quarter size sheet pan with 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 1-2 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. Transfer the cake batter into the lined sheet pan.
Bake for 25 to 30 minutes, or until the inserted toothpick comes out clean from the center of the cake pan. Cool the mini funfetti cake completely, before frosting.
To make mini funfetti cakes, once the cakes have cooled using a 2.5 or 3 inch circle cookie cutter. Cut out cicles. Once completed decorate in buttercream and sprinkles.