Preheat the oven to 350℉ (177℃). Prepare a 9x9 baking pan with nonstick cooking spray and parchment paper.
In a microwave-safe bowl, combine the unsalted butter and light brown sugar. Heat in the microwave for 90 seconds until the butter is fully melted. Whisk the butter and brown sugar together until combined and smooth. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment or in a large mixing bowl with an electric hand mixer, whip the granulated sugar and eggs on medium-high speed for a few minutes until light and pale in color.
Slowly pour in the brown sugar mixture into the egg mixture while mixing on medium speed to temper the eggs. Add in the vegetable oil and vanilla extract and mix until well combined.
Add in the all-purpose flour, dark cocoa powder, baking powder, and salt, and mix on low speed until the ingredients are well combined and the brownie batter is smooth. Scrape the sides and bottom of the bowl as needed. Fold in the roughly chopped Halloween Oreo cookies.
Pour the brownie batter into the prepared 9x9 baking pan. Use an offset spatula to spread the batter evenly in the pan. Press additional broken Halloween Oreo cookies into the top of the brownie batter.
Bake the brownies for approximately 30 minutes until the edges of the brownies are set and an inserted toothpick in the center of the brownies comes out with moist crumbs. Cool the brownies in the pan to room temperature.
To make the chocolate ganache drizzle, combine the semisweet chocolate chips and heavy cream in a microwave-safe bowl. Heat the mixture in the microwave in 15 second increments, mixing in between, until the chocolate is melted and the ganache is smooth.
Use a spoon or a piping bag to drizzle the chocolate ganache over the cooled brownies. Sprinkle the top of the brownies with Halloween sprinkles.