Preheat Oven and Prepare Pan:Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides to lift the brownies out easily once cooled. Lightly grease the parchment paper.
Make the Brownie Layer:In a large microwave-safe bowl, melt the butter in the microwave. Once melted, add the sugar and cocoa powder, and stir until well combined.Add the vanilla extract and eggs, one at a time, mixing well after each addition.In a separate bowl, whisk together the flour, salt, and baking powder. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Assemble the Brownie Base:Pour half of the brownie batter into the prepared baking pan and spread it out evenly.Sprinkle the chopped Oreo cookies evenly over the brownie batter.Pour the remaining brownie batter over the Oreos and gently spread it to cover them.
Add the Reese's Layer:Evenly distribute the chopped Reese's Peanut Butter Cups over the top layer of brownie batter, pressing them in slightly.
Bake:Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs attached. Be careful not to overbake.
Prepare the Fudge Topping:While the brownies are baking, heat the heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil).Remove from heat and add the chocolate chips, vanilla extract, and salt. Let sit for a minute, then stir until smooth and glossy.
Top the Brownies:Once the brownies are baked and slightly cooled, pour the fudge topping over the brownies, spreading it evenly with a spatula.
Cool and Set:Allow the brownies to cool completely in the pan, then transfer to the refrigerator for at least 1 hour to set the fudge topping.
Serve:Once the fudge topping is set, use the parchment paper overhang to lift the brownies out of the pan. Cut into squares and serve.