Line an 8x8 or 9x9 inch pan with aluminium foil and lightly grease, or line with parchment paper.
In a medium sauce pan over low heat melt together the butter, peanut butter and chocolate chips. Stir occasionally.
Remove from the heat and allow to cool.
Once the mixture is no longer hot, stir in the mini marshmallows. (If the pan is still hot, the marshmallows will melt.
Pour into the prepared pan and allow to set in the fridge for 2 hours.