In another bowl, mix the peanut butter with powdered sugar until smooth. Scoop tablespoons of brownie batter onto the prepared baking sheets. Drop half a teaspoon of peanut butter mixture on each and swirl gently with a toothpick.
6 Bake for 10-12 minutes, until tops are crackly and centers are soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack. Enjoy warm or store in an airtight container.