Grease and line the baking tin with baking parchment
Cube the butter and put it, along with the marshmallows, into a heavy based pan set over a medium heat. Stir frequently until everything has melted
Add the cereal and mix until thoroughly coated
Working quickly, tip the mixture into the baking tin and, using a piece of baking parchment, press it firmly down, so that it spreads into all of the corners and you get a compact, level surface. Set aside to cool completely (about 30 minutes)
Put all of the caramel ingredients into a heavy based pan set over a medium heat. Stir frequently until the sugar has dissolved
Turn up the heat slightly, and allow to boil gently until the temperature reaches 112°C – 115°C (234°F – 240°F). This is the soft ball stage. If you do not have a sugar thermometre, you can test for readiness by dropping a small amount of the caramel into some chilled water - it should form a firm ball that does not flatten when removed from the water, but squishes when squeezed
Once the caramel is ready, take the pan off the heat and plunge the bottom of it into cold water to stop the cooking process. Set aside and allow to cool for 10 minutes
Stir well, then pour over the cooled base. Set aside for at least 2-3 hours to firm up and cool completely
Finally, melt the chocolate and spread over the caramel layer. Allow to cool and set before cutting into squares (tip: if you can be patient, using a warm knife to score through the chocolate helps to stop the chocolate cracking in all directions and gives you neater squares).
Keep in an airtight container for up to 2 days