Line an 8x8 inch / 22x22 cm square tin with baking parchment and place to one side.
If you need to pop your popcorn then do so following the package instructions. Once done place in a large bowl and put aside for the moment.
Either in a microwavable bowl or a bain-marie carefully melt your chips, peanut butter and marshmallows. I did mine in a 800W microwave in 30 second bursts and found a little over 90 seconds did the trick.
Quickly mix everything together and pour it over your popcorn. With a quick but careful hand mix until all of the popcorn has a light coating.
If you are using the M&M's then add half of them now and give them a stir in.
Pour the mixture into your lined tin and pat it gently down so it all sticks together and sprinkle the remaining M&M's over the top and gently press them down to get them to stick.
Place in the fridge for 1 hour, then slice into either 9 or 12 squares depending what size you want.
Kept in an airtight tin at room temperature these will last up to 4 days. Kept in the fridge they should last for 7 days.