In a mixing bowl with a stand mixer, blend together the butter and sugars. Add the eggs one at a time, ensuring a thorough mix between additions. Stir in the vanilla.
Sift in the cocoa powder, all-purpose flour, cornstarch, baking soda, and salt. Gently mix until just combined, being careful not to overmix!
Take half of the cookie dough and spread it evenly in an 11×7 baking dish. Layer it with melted chocolate and peanut butter (or just peanut butter) along with peanut butter cups. Cover it with the remaining cookie dough and sprinkle peanut butter chips on top.
Bake at 350 degrees for 30 minutes, and voila! Enjoy your delightful, gooey stuffed cookie.