Line a 11x7 inch baking tray with some greaseproof paper and set aside.
In a large bowl, melt the butter/margarine and Mars Bars over a bain marie or in short bursts in a microwave.
Add the rice krispies to the melted ingredients and mix well, ensuring all are coated.
Tip into your prepared baking tray and push down with the back of a spoon, ensuring an even finish right into the corners. Leave to set in the fridge for at least 2 hours.
Once set, cut your Mars Bar Slice into your desired size and place on a fresh piece of greaseproof paper, with a little space between each.
Melt the white chocolate over a bain marie, or in short bursts in the microwave. Allow the chocolate to cool a little before drizzling over your Mars Bar slices. Finish with a couple of edible eyes, stuck on with some excess chocolate.
Allow the chocolate on your Mummy Mars Bar Slices to set for an hour or two, before enjoying.