Over a pan of simmering water, melt together the butter and Mars Bar chunks.
It will look like it's not going to melt at first but keep going until smooth.
Turn off the heat and stir in Rice Krispies until desired consistency.
Press into an 8in loose bottom cake tin and set in the fridge.
For the filling, beat together cream cheese, sugar and boiling water until smooth.
Melt the chocolate and pour over heavy cream and continue to beat until well combined.
Fold in chunks of Crunch Bar.
Pour over base and leave to set in the fridge.
To decorate, garnish with Crunch Bar and drizzle with caramel sauce.