Melt the Mars bars and butter in a bowl set over just simmering water. Don’t let the bowl touch the water. When fully melted, the mixture should come together into a smooth, thick paste. Take the bowl off the heat and stir in the Rice Krispies. Divide into six equal bars and shape with buttered hands or silicone gloves if you have them.
Leave to firm up while you melt the caramels and milk in a small non-stick saucepan or in the microwave. Stir the mixture together then spoon it on top of the bars, smoothing it on as you go.
Line a tray with greaseproof paper. Take one bar and submerge it in the melted chocolate. Use two forks to lift it out and drain any excess chocolate. Transfer to the lined tray, repeat with the rest and leave to cool for around 1 hour until set.