Preheat oven to 350°F (175°C). Line an 8×8-inch or 9×9-inch baking pan with parchment paper.
In a large bowl, mix melted butter, sugar, eggs, and vanilla until smooth and slightly fluffy.
Add flour, cocoa powder, and salt. Stir until just combined.
Fold in whole Oreo cookies (keep some for topping if desired).
Pour half the batter into the pan. Place additional Oreos on top, then cover with remaining batter.
Optional: Swirl in cream cheese mixture (mix 4 oz softened cream cheese + 1/4 cup powdered sugar + 1/2 tsp vanilla) for extra richness.
Bake for 25–30 minutes or until the edges are set but the center is still fudgy.
Let cool completely before slicing into squares.