Preheat oven to 350°F (175°C) and grease an 8x8 inch baking pan with butter or non-stick spray.

In a medium mixing bowl, whisk together the melted butter and granulated sugar until well combined.

Whisk in the eggs and vanilla extract until the mixture is smooth and slightly glossy.

Sift the cocoa powder, flour, salt, and baking powder directly into the wet ingredients.

Gently fold the dry ingredients into the wet ingredients using a spatula until just combined; do not overmix.

Spread approximately half of the brownie batter into the bottom of the prepared baking pan.

Place 16 Oreo cookies in a 4x4 grid on top of the batter layer.

Pour the remaining brownie batter over the cookies and spread evenly to the edges, ensuring the cookies are covered.

Evenly distribute the Halloween sprinkles over the top of the batter.

Bake for 30 to 35 minutes, or until a toothpick inserted into the center (avoiding an Oreo) comes out with a few moist crumbs.

Remove from oven and allow to cool completely in the pan before slicing into 16 squares.