Line a large rectangular baking tin with greaseproof paper.
Melt the butter and mini marshmallows together in a large bowl set over a pan of simmering water.
Remove from the heat, then stir in the Rice Krispies and salt, coating them in the mixture.
Spoon into the baking tin and firmly press out into an even layer.
Set aside and leave to set for 1 hour, then lift out of the baking tin and slice into small rectangles.
Divide the icing sugar between three small bowls. Add a few drops of water to each bowl and stir together to make a thick icing.
Add a drop of food colouring to dye each bowl of icing a different colour.
Spoon the icing over the top of each Rice Krispie rectangle and decorate with edible eyes. Leave to set.