Preheat the oven to 350°F (175°C) and line an 8×8-inch or 9×9-inch baking pan with parchment paper or grease it lightly.
Whisk melted butter, egg, brown sugar, granulated sugar, and vanilla until smooth and glossy.
In another bowl, mix flour, baking powder, and baking soda, then stir into the wet mixture until just combined.
Fold in crushed Oreos, white chocolate chips, and chopped Hershey’s bar, saving a few for topping.
Spread the batter evenly in the pan, sprinkle the reserved toppings, and bake for 20–23 minutes until golden at the edges and slightly soft in the center.
Cool for 5 minutes, lift out, and transfer to a wire rack. Once cooled, slice into 16 squares.