Chop the butter and Mars bars into small pieces and place in a large heatproof bowl.
Place a little water in a saucepan and simmer.
Suspend the bowl over the simmering water (not allowing the base of the bowl to touch the water) and melt the Mars bars and butter, stirring firmly until completely blended.
Take the bowl off the heat and add the Rice Krispies, and mix thoroughly.
Place in a 30cm x 20cm x 5cm tin.
Melt the chocolate over the still simmering water.
Pour the chocolate over the mixture, spread evenly and allow to set.
Turn out of tin and cut into squares.