Preheat oven to 350º F. Grease mini muffin tins very well.
Fill muffin tins with brownie batter.
Bake for 10 minutes. The centers should cave in a little once you remove from the oven. Allow to cool completely.
Mix together peanut butter, sugar, butter and vanilla until smooth. Fill center of brownies with 1 heaping teaspoon of the peanut butter mixture.
Top with a few chocolate chips.
