Use cooking spray to spray two bread tins or similarly sized trays.

In a large microwave-safe bowl, add the butter and marshmallows. Blast for 90 seconds and stir. Blast for 30 more seconds, stir and repeat once more. Keep your eye on the microwave the whole time to ensure the marshmallow mixture doesn’t overflow.

Remove from the microwave using a tea-towel, mix in the vanilla and thoroughly stir through the cereal.

Divide the mixture into your prepared pans and use your hands to compress it down to create a dense bar. Watch out, it'll still be warm at this point!

Allow to sit in the fridge for 30 minutes before slicing into your desired shape and serving. Eat within 48 hours.