Cut the Mars Bars up into small pieces.
Add chopped Mars Bars and butter to a saucepan.
Stir over low heat until melted and combined.
Remove from heat and add the Rice Bubbles/Puffs.
Mix well.
Press into the base of a lined, square baking tin.
While the slice is left to set, melt the chocolate by breaking into pieces and adding it to a microwave safe bowl.
Heat chocolate on high in the microwave for 30 seconds.
Stir.
Heat for another 30 seconds and stir again.
Repeat again if chocolate has not fully melted, but reduce time in microwave to only 10 second intervals to make sure you do not overcook or burn your chocolate.
Once chocolate has melted, pour over the top of the slice base and spread.
Place slice into the fridge to set.
Slice and enjoy.