Line an 8 inch baking pan with foil or parchment and set aside.
First make the cookies by combining the butter and sugar. Add the vanilla and egg.
Add the flour, cornstarch, baking soda and salt. Stir to combine then add the sprinkles. Chill the dough in the fridge while you prepare the brownie mix. Keep in mind you will not be using all the cookie batter for this recipe. See notes
Preheat the oven to 325 degrees
Once the brownie mix is ready, pour half of the batter in the prepared pan. Add small chunks of the cookie batter then pour the remaining brownie batter over top. Add more small chunks of cookie batter then bake for 40 minutes.
Cool completely before serving.
Notes
To make sprinkle cookies, preheat the oven to 350 and bake the cookies for 10 minutes then leave them on the sheet for about 5 minutes before transferring to a cooling rack. Store in an airtight container.
Recipe by Friday is Cake Night at https://www.fridaycakenight.com/2018/02/birthday-brownies/