Preheat oven to 350 degrees F. Grease a 9x9 pan with cooking spray; set aside.
In a small saucepan, melt the peanut butter chips and butter together over low heat. Stir until smooth. Remove the pan from the heat and let cool for 5-10 minutes.
In a large bowl, combine the brown sugar, eggs and vanilla. Stir in melted peanut butter. Add flour, baking powder, and salt. Stir until combined. Stir in the marshmallows and chocolate chips.
Spread the batter evenly into the prepared pan.
Bake for about 20 minutes. Remove from oven and let cool completely.
Cut into squares and enjoy!