Spray a 9 X 13 inch baking pan with baking spray.
Layer the pan with the marshmallows.
Melt the chocolate chips, peanut butter, and butter in the microwave on medium (50% power) for 2 to 3 minutes.
Stir to combine.
Allow to cool for at least 10 minutes.
Stir in the vanilla.
Pour the chocolate mixture over the marshmallows and stir to distribute.
Spread evenly in the pan.
Cool in the refrigerator or freezer for at least 1 hour and up to overnight before cutting into squares.