For cookies:
1 cup unsalted butter softened, 226g
¾ cup brown sugar 150g
1/2 cup granulated sugar 100g
1 egg
2 tsp. vanilla extract 10mL
2 cups all-purpose flour 240g
1 ¼ tsp. kosher salt
½ tsp. baking powder
¼ tsp. baking soda
1 cup semi-sweet chocolate chips
½ cup quick oats

for brownies:
3/4 cup unsalted butter melted
½ cup brown sugar 100g
½ cup granulated sugar 100g
2 tsp. vanilla extract 10mL
2/3 cup cocoa powder 65g
½ cup all-purpose flour 60g
2 tsp. espresso powder
½ tsp. kosher salt
½ tsp. baking powder
2 eggs
½ cup semi-sweet chocolate chips

Make the Cookie Dough:
Using a stand or hand mixer, beat butter and sugars in a bowl until creamy, about 2 minutes. Beat in egg and vanilla until combined.
In a separate bowl, stir together flour, salt, baking powder, and baking soda. Gradually add flour mixture to butter mixture, beating on low speed until just combined.
Fold in both chocolates and oats until just combined (do not overbeat).
Divide dough in half; spread half of dough in an even layer in the bottom of a foil-lined, greased 9x9-inch baking pan. Set pan aside while making brownie layer.
Refrigerate remaining dough for 15 minutes; reserve for additional cookies.

Make the Cookie Layer:
Place the Oreos on top of the cookie dough layer; breaking cookies to fit, as needed.

Make the Brownie Layer:
Preheat oven to 350°F. Mix the cocoa powder, flour, espresso powder, and baking powder in a bowl then set aside.
In a medium bowl, beat together melted butter, sugars, eggs, and vanilla. Mix very well as this step gives you that nice brownie top.
Sift the dry mixture into the butter mixture; stir until no streaks remain then fold in the chocolate chips.
Pour brownie batter over Oreo layer; gently smooth top with an offset spatula.
Bake at 350° until a wooden pick inserted comes out mostly clean, 35 to 40 minutes.
For best slicing results, let cool completely, about 2 hours.
Using foil as handles, lift brownies from pan. Discard foil; cut brownies into desired shapes.