Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy removal.
Melt the butter in a microwave or saucepan. Let it cool slightly.
In a large bowl, whisk together butter, sugar, eggs, and vanilla until smooth and glossy.
Sift in cocoa powder, flour, salt, and espresso powder (if using). Gently fold until just combined.
Stir in chocolate chips and half of the chopped Oreos.
Spread the batter evenly into the prepared pan. Top with the remaining Oreos, pressing them gently into the surface.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool in the pan for at least 30 minutes before lifting out and slicing into squares.