Preheat your oven to 350 degrees Fahrenheit and line a 9x13-inch baking dish with parchment paper.

Use a rubber spatula and a medium mixing bowl to combine the cookie mix with the semi-sweet chocolate chips for your cookie layer.

Gently pour in the melted butter and egg, and mix thoroughly. The dough will feel slightly sticky.

Press the prepared cookie dough carefully into the bottom of your prepared pan. Lightly floured fingers or a wet spatula will work best for this step.

For your chocolate sandwich cookie layer, open your package of Double Stuf Oreos and place them in rows over your prepared cookie dough in an even layer. You may have to break some apart to get them to fit.

Set the pan with the finished two layers aside.

Pour your brownie mix into a medium mixing bowl and stir in the chocolate chips using a rubber spatula.

Add in your eggs, butter, and milk and stir to combine. Depending on your brownie mix, you should have a regular to thick brownie batter. Be sure to incorporate everything together, scraping down the sides as you go.

Take your brownie mixture and gently pour over the sandwich cookies, taking your time to cover the cookie layer in your baking dish. Use a spatula to even the brownie batter into an even layer if necessary.

Place the baking dish into the oven and cook for 50 minutes or until the top has set and a toothpick inserted in the middle comes out clean.

Let cool completely, about 30-60 minutes, before slicing into squares for best results!