Line a large baking tin with overlapping pieces of clingfilm, ensuring that all parts of the interior are covered.
Place the butter and sliced Mars bars in a large Pyrex (glass) bowl and place this into a suitably-sized saucepan (the bowl should not touch the bottom, and leave space for 2 inches of water). Bring the water to a strong simmer and melt the butter & Mars bars, stirring occasionally. Once it has all melted, stir continuously until the the melted butter is absorbed into the rest – be patient, it will “come together” eventually.
Lift the bowl off the saucepan and add in 5 mugfuls of Rice Krispies. Stir this well, but gently, until all the Rice Krispies are coated and no lumps of the melted mixture remain. Pour / spoon into your baking tin and gently smooth out until even.
Wash your Pyrex bowl, dry and place back on the saucepan of water. Break up the chocolate into the bowl and melt over simmering water. Once melted and smooth, pour over the Rice Krispies. It’s best to use a soft spatula so that you can scrape out as much of the chocolate as possible. Tip the baking tray back and forward until the base layer is evenly covered.
Cover with a tea towel and leave in a cool place to set (varies from 2 – 24 hours, dependent on season/temperature). Do NOT refrigerate as this will make the Rice Krispie layer go soggy.
Cut with a sharp knife and store in an air tight container (old biscuit tins are perfect), will keep for up to 7 days – but I bet they won’t last that long!