Begin by preheating your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by either greasing it with non-stick spray or lining it with parchment paper.
In a large mixing bowl, combine the melted butter and sugar. Mix until smooth and fully integrated.
Add the eggs one at a time to the butter and sugar mixture, mixing thoroughly after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Gradually incorporate the dry mixture into the wet mixture, stirring gently until just combined.
If using, gently fold in the chocolate chips, ensuring they are evenly distributed.
Pour half of the brownie batter into the prepared baking pan and spread it evenly.
Place the split Oreo cookies on top of the brownie batter, cream side facing up, and gently press them down.
Pour the remaining brownie batter over the Oreos and spread it evenly to cover the cookies.
Bake the brownies for 25-30 minutes. Check for doneness with a toothpick; it should come out with a few moist crumbs.
Allow the brownies to cool in the pan for about 20 minutes before cutting them into squares.
Notes
For an indulgent touch, serve warm brownies with a scoop of vanilla ice cream and drizzle with chocolate syrup.