Melt the Mars bars and butter together in a pot, then mix the Rice Krispies in with them, coating the cereal completely.
Transfer to a well greased 9Γ9 or 8x8 pan.Pat into an even layer.
Melt the chocolate and butter for the topping, stirring until smooth, then pour it on top.
Cool in the fridge briefly to set the topping, then cut into pieces and serve.
Store in a closed container at room temp for up to 4 days.