Preheat the oven to 350°F.
Line a 9 x 13-inch rectangular baking pan with parchment paper and spray with nonstick cooking spray.
Beat together melted butter and sugar with a hand mixer or stand mixer.
Add vanilla extract and eggs, adding one egg at a time, beating well between each addition.
Sift together flour, cocoa powder, salt, and baking powder then add to the butter/sugar mixture.
Beat until fully combined.
Spread half of the batter across the bottom of the prepared baking pan. The batter will be thick in texture and will need to be spread carefully to cover the entire pan.
Nestle the chocolate peanut butter cups into the brownie batter, forming neat rows.
Spread the remaining batter over the peanut butter cups and smooth evenly.
Bake for 30 minutes. Batter should be set in the middle and a toothpick inserted in the center should come out with a few moist crumbs.
Remove from the oven and allow brownies to cool completely in the pan.
Brownies will lift easily from the pan by lifting on the edges of the parchment paper lining.
Cut into pieces and serve!