Preheat oven to 375°. Line a muffin pan with 12 paper liners.
In a medium bowl, whisk together the flour, baking powder, salt and brown sugar. Set aside.
In a large bowl, whisk together the melted butter, peanut butter, eggs and milk until smooth. Add in the flour mixture and stir until just combined. Stir in chocolate chips. Divide batter evenly between prepared muffin tins, filling each to the top.
Bake for 17-20 minutes or until a toothpick inserted in the center comes out clean. Allow to cool for 5 minutes in the pan, then remove to a wire rack to cool completely.