Bake brownie cookies according to the recipe. Note-- the chocolate chips in the recipe are optional and we make these cookies slightly larger than the Fudgy Brownie Cookie recipe calls for. Allow cookies to cool completely on a cooling rack before frosting.
While the cookies are baking, prepare the buttercream frosting in an electric stand mixer. Refrigerate frosting while the cookies cool.
Transfer frosting to a 16" piping bag with a 2A round tip, Start in the middle of the cookie and pipe the buttercream frosting in a swirl shape until you get to the outside of the cookie, as pictured.
Or use an angled spatula and evenly frost each cookie.
Top cookies with crushed Oreo cookies and serve.
Notes-- buttercream frosting can made and refrigerated days in advance. As you pipe the buttercream frosting onto the cookie, the frosting might soften from your handgrip. Place the piping bag and tip into the refrigerator for 10 minutes to harden and resume decorating.After cookies have been decorated, store them in an airtight container in the refrigerator.