Line a tray bake tin with baking paper (allowing enough paper to go over the top of cake)
Weigh out butter and put in a pan on a low heat
When the butter is melted add marshmallows
Keep stirring on the lowest heat, making sure it does not burn (you may even need to remove the pan from heat at times, depending on hob). It will melt fairly quickly, you just need to wait and not be tempted to turn up heat
When melted take the pan off the heat and add approx 5 cups of rice crispies. Add 1 cup at a time and stir. It will cool quickly when it is off the heat so you need to get rice crispies stirred in quickly.
Pour rice crispies in to the lined tin.
It is very sticky so the easiest way to compact rice crispies is by folding over excess baking paper and pressing and fold back straight away to stop it sticking.
Add decorations of choice
Leave to set at room temperature for 20-30 minutes
Remove from tin and peel down baking paper
Cut in to squares (best eaten fresh, but if stored put baking paper between to stop them sticking and keep at room temperature)