Grease a round tin (with a clip is preferred). A smaller tin means a thicker layer of chocolate, and a larger one means thinner layer of chocolate.
In a sauce pan, place the butter, caster sugar and syrup. Let this completely melt but don't let it burn, stirring occasionally. Add the drinking chocolate and stir in until completely combined.
Whilst still hot add in 100g of the marshmallows and let them melt into the mixture, stirring constantly. The mixture should become thicker. Check the mixture is Sweet enough and if not add a few more tsp. of sugar.
Let the mixture cool. In a bag place half of the biscuits and use a rolling pin to crush, to do this I use the end and then roll the rolling pin over the bag. You want the biscuits to have some small bits and some crumbs. When finished do exactly the same with the other half.
To the chocolate sauce mix add the biscuits and mix until the biscuits are evenly distributed. Now cut the malteasers in half and add them to the mixture.
Tip the biscuit mixture into the prepared tin. Using the back of a spoon evenly spread the mixture to the sides of the dish making sure there are no gaps. Now put this into the fridge whilst preparing the topping.
To melt the chocolate. Break it into even sized pieces. Heat a pan of water and place a glass bowl over the top of the water but make sure the bowl does not touch the water. Add small amounts of chocolate at a time and let it melt stirring occasionally. Keep doing this for all the chocolate, when all is melted get the biscuits out of the fridge.
Take the rest of the marshmallows and sprinkle over the top of the biscuit mixture. Pour the melted chocolate over the chilled mixture and spread over evenly with a spatula. Let this chill for around 2 hours or until chocolate is firm.
When chocolate is hard. Cut the tiffin into pizza like shape slices. You can present it on a cake stand or just on a plate. Now enjoy your tiffin with a cup of tea and a good old TV programme. Enjoy :)