Grease a 20cm/8in square, deep-sided tin with butter.
Suspend a large heatproof bowl over a pan of simmering water (do not allow the base of the bowl to touch the water).
Add the Mars bars and butter to the bowl and stir until melted, smooth and well combined. The mixture will curdle at first, but keep stirring (or use a whisk) and it will eventually become smooth.
Remove the bowl from the heat, add the Rice Krispies and fold until coated in the chocolate mixture.
Spoon the mixture into the prepared tin and press down to form an even layer, pushing the mixture into the corners and sides of the tin. Set aside to cool.
Meanwhile, suspend another heatproof bowl over the pan of simmering water (do not allow the base of the bowl to touch the water). Melt the chocolate in the bowl, then drizzle or spread it over the cooled chocolate crispy cake. Set aside to cool and harden.
Chill the chocolate crispy cake in the fridge until set, preferably overnight. Cut into squares while still in the tin, then remove from the tin, pile onto a plate and serve.