Line a half sheet pan (15x10x1) with aluminum foil or parchment paper and spray with cooking spray.
In the bowl of your stand mixer, cream together the butter, cream cheese, and sugar. Beat in the egg and vanilla until well incorporated.
With your mixer turned to low, add in the flour, baking powder, and baking soda, mixing until just combined. Stir in the sprinkles.
Press the dough evenly into your prepared pan. The dough is sticky and it will look too thin, but it all works out. Just keep pressing it until you have an even layer over the entire baking sheet.
Bake for 20 minutes until edges begin to turn golden.
Cool completely before frosting and topping with additional sprinkles.