Preheat your oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing it lightly.

Combine melted unsalted butter, granulated sugar, and vanilla extract in a medium bowl until blended.

Incorporate eggs one at a time, mixing thoroughly after each addition.

Whisk together all-purpose flour, unsweetened cocoa powder, baking powder, and salt in a separate bowl.

Gradually add the dry ingredients to the wet mixture until just combined.

Gently fold in the crushed chocolate sandwich cookies and semi-sweet chocolate chips if using.

Pour the brownie batter into the prepared baking pan and spread it evenly.

Bake for 30-35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.

Let the brownies cool in the pan for about 10-15 minutes before transferring to a wire rack to cool completely.

Slice into squares and serve as desired.