Step 1

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.

Step 2

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.

Step 3

Fold in both chocolates and oats until just combined (do not overbeat).

Step 4

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.

Step 5

Refrigerate remaining dough for 15 minutes; reserve for additional cookies (see note below).

Step 6

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

Step 7

Preheat oven to 350°F. Mix the cocoa powder, flour, espresso powder, and baking powder in a bowl then set aside.

Step 8

In a medium bowl, beat together melted butter, sugars, eggs, and vanilla. Mix very well as this step gives you that nice brownie top.

Step 9

Sift the dry mixture into the butter mixture; stir until no streaks remain then fold in the chocolate chips.

Step 10

Pour brownie batter over Oreo layer; gently smooth top with an offset spatula.

Step 11

Bake at 350° until a wooden pick inserted comes out mostly clean, 35 to 40 minutes.

Step 12

For best slicing results, let cool completely, about 2 hours.

Step 13

Using foil as handles, lift brownies from pan. Discard foil; cut brownies into desired shapes.

Step 14

Store in an airtight container up to 3 days, or refrigerate up to 1 week, if you can resist them that long!