Measure out the Rice Krispies and set aside in a large mixing bowl.
Take a small oven dish (about 9 x 5 inch) and grease well or line with greaseproof paper.
Melt the butter in a saucepan on a low heat.
Add the marshmallows and stir constantly until melted and well combined with the butter.
Pour your mixture over the Rice Krispies and stir together quickly before it cools.
Straight away, spoon the mixture out onto your greased baking dish.
Make a level layer about 2 cm deep and press it with the back of a spoon. If the mixture is too sticky and stringy to do this, then place a sheet of greaseproof parchment paper over the Rice Krispie mixture and push down with your hands until compacted. They won’t stick to the parchment paper and you will end up with nice thick bars.
Place in the fridge to chill for 2 hours or overnight.