Preheat the oven to 350 F. Line an 8×8 inch baking pan with parchment paper or grease the bottom of the pan, or spray the pan with non-stick cooking spray. Set it aside.
Place the butter in a large microwave safe mixing bowl. Heat the butter until it is completely melted, wait a few minutes for it to cool before adding the additional ingredients.
Add the cocoa powder and sugar to the melted butter and whisk until well blended
Add in the eggs, vanilla extract, and salt and whisk just until the eggs have been fully incorporated into the batter. Do not over mix.
In a separate bowl mix together the flour and baking powder.
Add half of the flour mixture into the chocolate mixture and stir to combine. Add in the remaining flower and continue to stir until everything is well mixed.
Gently fold in half of the peanut butter cups and peanut butter chips. Set the remaining aside to top the brownies.
Spread the brownie batter into the prepared pan in an even layer.
Spread the reserved mini peanut butter cups and peanut butter chips over the top of the brownie batter and gently press them down.
Place the baking pan in the oven and bake at 350 F for 23-27 minutes, or until a toothpick inserted into the center of the brownies comes out mostly clean. For extra fudgy brownies, cook on the lower end of the bake time or remove the brownies from the oven a minute or two early.
Place the baking pan on a wire rack to cool completely before cutting into 9 squares and enjoying!
Store your brownies in an airtight container.