Melt the butter over a medium heat in a large saucepan. Add the large marshmallows and stir regularly until melted.
Pour the cereal into a large mixing bowl.
Once the marshmallow butter mixture has fully melted, pour it over the cereal in the bowl. Mix together and pour into a rectangular foil baking tray or similar.
Melt the chocolate in a heatproof bowl set over a pan of gently simmering water, ensuring the bowl doesn't touch the water. Once melted and smooth, remove from heat.
Sprinkle the mini marshmallows over the cereal mixture.
Liberally drizzle the melted chocolate over the top and put in the fridge to set.