Preheat oven to 350°F.Line an 8-inch square baking pan with parchment paper, or grease with butter or non-stick cooking spray. Set aside.
Add butter to a microwave-safe bowl. Microwave on high power for about 1 minute, or until the butter melts.
In a large mixing bowl, whisk to combine melted butter, sugar, eggs, and vanilla extract.
To the butter mixture, add cocoa powder, flour, salt, and baking powder. Stir to combine, until a thick batter forms.
Fold in chopped peanut butter cups. If desired, save some to sprinkle over the brownie batter before baking.
Pour brownie batter into prepared baking pan. Bake in preheated oven for 20-22 minutes, or until edges are firm but center is slightly undercooked. (The brownies will continue baking as they cool).
Notes
For cleaner slices, let the brownies cool before cutting into squares.
Storing: This dessert will stay fresh for about four days in an airtight container at room temperature.Freezing: You can freeze the cocoa brownies for up to 3 months in an airtight container. It's best to freeze individual servings, separated with squares of parchment paper.Thaw completely before enjoying.