Microwave the milk, butter and chips for 1 minute at 70% power.
Stir until smooth adding 15 seconds at a time back in the microwave if not smooth.
Butter the bottom of an 8x8 pan and line with parchment. The butter will keep it in place.
Add the frozen peanut butter cups and immediately pour in.
Pop in freezer 10 minutes and then move to the fridge for several hours.
Cut and eat, eat, eat!