Preheat the oven to 350º F. Spray a 9x13 baking dish with nonstick baking spray, such as Baker's Joy. Set aside.
Chocolate Chip Cookie Layer
Cream butter then add sugars and mix until light and fluffy. Add eggs and vanilla. Mix until well-combined. Add flour, oats, salt, and baking soda to butter mixture. Mix well and then stir in chocolate chips. Spread into the prepared 9x13 baking dish. Set aside.
Fudge Brownie Layer
Melt butter in a large saucepan over low heat. Stir in cocoa powder, espresso powder, and salt until well combined. Remove from heat.
Stir in both sugars and then the flour. Add eggs, one at a time, mixing until just blended. Stir in vanilla and nuts, if using. Pour into the brownie batter over the chocolate chip cookie layer in the prepared baking pan.
Bake for 25 minutes.
Peanut Butter Cup Layer
Add the unwrapped peanut butter cups to the top of the nearly fully baked brownie layer. Return to the oven and bake until a toothpick inserted into the the brownie portion of the brookies comes out with a few crumbs attached.
Allow the brookies to cool in the pan over a wire rack for about 15 minutes before slicing. Slice into 12 or 24 pieces for serving.
You can also use 36 miniature Reese's Peanut Butter Cups.