Preheat oven to 350 F. Line an 8x8" square nonstick pan with parchment paper.
Spread chocolate chip cookie dough into a 1/4 inch layer on the bottom of the pan.
Place Oreo cookies in an even layer on top of the cookie dough. You can fill in the gaps with circus animal cookies if you're using them!
Pour a thin layer of brownie batter on top of the Oreo Cookies so that they are just covered (if the layer is too thick, the brownies will take much longer to cook through) Top this brownie layer with rainbow sprinkles if desired.
Place in the oven for 30-35 minutes. Use a toothpick to check if the brownies are cooked through before removing from the oven, as cooking time will vary depending on the thickness of your layers.
Once the brownies are out of the oven, quickly arrange Circus Animal Cookies on top if you'd like, pushing them down lightly into the brownie layer so that they stick. If not, let cool slightly, then cut, serve and enjoy!