Preheat the oven to 350°F and lightly grease the bottom of an 8x8-inch baking pan and line it with parchment paper. You can ball the parchment paper up and run it under water for a few seconds to soften it up and make it easier to work with. Just pat it dry with a paper towel before putting it in the pan. Set it aside.
In a large microwave-safe bowl, melt the ½ cup unsalted butter and allow it to cool for 5 minutes before adding the other ingredients.
Once the butter is cooled, whisk in ¾ cup granulated sugar and ½ cup unsweetened cocoa powder.
Add in the 2 large eggs, 1½ teaspoons pure vanilla extract, and ¾ teaspoon salt just until the eggs have been fully incorporated into the batter.
Mix in the 1¼ cups all-purpose flour and ¼ teaspoon baking powder until well combined, but try not to overmix.
Fold in ¾ cup of the 1 cup Reese’s peanut butter cup minis and ½ cup of the ¾ cup Reese’s peanut butter chips. Reserve the rest for topping.
Spread the brownie batter into the prepared baking pan in an even layer.
Add the remaining Reese’s peanut butter cup minis and chips for topping, gently pressing them into the batter.
Bake for 22 to 26 minutes, or until a toothpick inserted in the center comes out with just a little bit of crumbs.
We recommend letting the brownies cool in the pan on a wire rack completely before cutting them into squares and serving. Cutting with a plastic knife will help ensure clean cuts.