My cake used 4 mars bars, melted over a low heat in a small saucepan with a hefty dollop of golden syrup. When it was all melted and deliciously gooey I added the rice crispies, stirred to cover them all with the gooey goodness, and then added in smarties, chopped marshmellows, and some chopped up galaxy ripple.
I poured the mixture into a heart shaped silicone mould and whacked it into the fridge to set. The heat of the chocolate and crispy mixture melted a lot of the marshmellow, so when I cut into the cake later it was very, very sticky and gooey. But oh so nice!