Begin by preheating your oven to 350°F (175°C). Grease a 9x13 inch baking dish thoroughly, then line it with parchment paper, allowing some overhang on the sides to facilitate easy removal after baking.
In a large mixing bowl, combine the melted butter with the granulated sugar and packed brown sugar. Whisk them together until the mixture is smooth and well blended.
Incorporate the eggs into the mixture one by one, ensuring to mix thoroughly after each addition. Add the vanilla extract and stir until fully combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, salt, and baking powder. Gradually fold this dry mixture into the wet ingredients, mixing just until no dry streaks remain. Avoid over-mixing to keep your brownies tender.
Carefully pour half of the brownie batter into your prepared baking dish, spreading it evenly across the bottom to form a base layer.
In a small bowl, if needed, warm the peanut butter slightly with a touch of the melted butter to make it more spreadable. Drop spoonfuls of the peanut butter mixture over the brownie base, spreading it gently to create an even layer.
Scatter the chopped mini peanut butter cups evenly over the peanut butter layer for delightful bursts of flavor in every bite.
Gently pour the remaining brownie batter over the top, using a spatula to ensure the mini peanut butter cups are completely covered.
If you love a rich chocolatey finish, sprinkle the chocolate chips on top of the final layer of brownie batter.
Bake the assembled brownie bars in your preheated oven for 25-30 minutes. Check for doneness by ensuring the edges are set while the center remains soft (a toothpick inserted may come out with a few moist crumbs).
Once baked, remove the dish from the oven and allow the brownie bars to cool in the pan for approximately 20 minutes. Optionally, right before they cool completely, sprinkle a pinch of sea salt on top for a contrasting flavor.
Carefully lift the cooled brownies out of the pan by pulling on the parchment overhang. Transfer them to a wire rack to cool completely before slicing into bars.
Notes
For a stunning display, serve the brownie bars stacked on a tiered cake stand or arrange them on a rustic wooden board.