Heat oven to 350°F. Grease an 8-by-8-inch baking pan and line with a strip of parchment, allowing edges to hang over opposite sides.
For the chocolate chip cookie layer, beat together the butter, granulated sugar and brown sugar. Add egg and vanilla; beat until fluffy. Stir in the flour, salt and baking soda, then add chocolate chips. Press dough into the bottom of the pan. Top with a single layer of Oreos.
To make brownie batter, melt together butter and chocolate and cool slightly. Stir in brown sugar, vanilla and eggs until smooth. Add flour, cocoa powder, salt and baking powder and stir until batter is smooth and glossy. Pour over Oreos in the pan.
Top brownie layer with extra crumbled Oreos. Bake until a toothpick inserted into the center comes out almost clean, 60 to 65 min. Allow to cool, then run a spatula around edges to loosen from pan. Use the parchment overhang to lift brownies out of the pan; slice and serve.
Nutter Butter-PB Variation
Substitute smooth peanut butter for ½ cup of the unsalted butter in the chocolate chip cookie recipe. Make as directed otherwise. Use Nutter Butter cookies in place of the Oreos and sprinkle peanuts on top of the brownie layer.
Mint and Milano Variation
Substitute chopped Andes Mints for ½ cup of the mini chocolate chips in the chocolate chip cookie recipe. Make as directed otherwise. Use Pepperidge Farm Milano Cookies in place of the Oreos and sprinkle extra chopped Andes Mints on top of Brownie layer just after baking.
You can also make Nutter Butter-peanut butter or Andes Mint-Milano cookie variations of these bars.