Preheat your oven to 350°F (175°C). Line your 9x13 inch baking pan with parchment paper.
Melt 1 cup of butter in a medium saucepan over low heat. Remove from heat and let cool slightly.
In a large mixing bowl, combine melted butter and 2 1/4 cups of granulated sugar. Whisk in 4 eggs one at a time until smooth. Stir in 2 teaspoons of vanilla extract.
In a separate bowl, whisk together 1 1/4 cups of cocoa powder, 1/2 teaspoon of salt, 1 teaspoon of baking powder, and 1 1/2 cups of flour. Gradually fold into the wet mixture.
Gently fold in 1 cup of chocolate chips until evenly distributed.
Spread half of the brownie batter into the prepared pan. Layer with Oreos, then cover with remaining batter.
Bake for 30-35 minutes until a toothpick inserted comes out with moist crumbs.
Cool in the pan for 10 minutes, then lift out using parchment paper. Transfer to a cooling rack and decorate with frosting once cooled.
