Line a metal 8-inch square pan with parchment paper, leaving an overhang. Spread the peanut butter into an even layer in the pan. Freeze for 1 hour or until solidified. Use the parchment to remove from pan and return peanut butter sheet to freezer. Keep frozen while making the brownie batter.
Brownies can be stored in an airtight container at room temperature for up to 3 days or in the fridge for up to 5 days. Serving chilled makes the brownies extra fudgy!
Feel free to reduce the amount of peanut butter to 1 cup or 3/4 cup if you want less.