Preheat the oven to 350°F (175°C) and grease a baking pan.
In a mixing bowl, combine melted butter and sugar until smooth.
Add eggs one at a time, mixing after each addition, then stir in vanilla extract.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the chopped Oreos and chocolate chips (if using).
Pour the batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick comes out with moist crumbs.
Allow the brownies to cool in the pan for 15 minutes before transferring to a wire rack.
Once cooled, cut into squares and serve.