Melt 4 tablespoons of butter in a large saucepan over medium-low heat. Stir in mini marshmallows until completely melted, then remove from heat.

Quickly add in half of the crushed Oreo cookies and the rice cereal. Stir until everything is well coated with the marshmallow mixture.

Spread half of the cereal mixture in a greased 9x9 baking dish.

Layer on the marshmallow fluff and sprinkle pieces of the Hershey's chocolate bar and the remaining crushed Oreo cookies on top.

Add the rest of the cereal mixture on top, spreading it out until smooth. Gently press Oreo chunks into the treats.

Let it cool at room temperature to solidify. Then, slice and serve!