Break the biscuits into small pieces and place in a bowl.
Add the sultanas and cocoa powder and mix.
Measure the butter, Demerara sugar and honey into a saucepan. Heat gently until melted and the sugar has dissolved.
Add the butter/sugar/honey mixture to the dry ingredients and mix well.
Line a 20cm by 30cm (or equivalent) tin with non stick parchment.
Spoon the tiffin mixture into the lined tin, pressing firmly with the back of a spoon.
Place in the freezer for about 15 minutes then transfer to the fridge until it has chilled and set (about 2 hours total).
Remove from the tin, remove the lining paper and place on a place or board.
Melt the dark and white chocolate.
Coat the top of the tiffin with the dark chocolate.
Using a teaspoon drizzle white chocolate over the dark chocolate. Drag a skewer through the white chocolate drizzles to create a marbled effect (before the chocolate sets).
Notes
Tips
When breaking the biscuits make sure some small pieces remain to add texture to the tiffin.
If you want to save time you could cover with a simple chocolate topping rather than a marbled topping.
Cut into squares before refrigerating. When the chocolate has been chilled it is more likely to crack when it is cut.
Storage
Store the chocolate tiffin cake in an airtight container in a cool place where it will keep for several days. Alternatively, store in the fridge, particularly during hot weather.
The tiffin can also be cut into portions and frozen either in a plastic tub or sealed bags, where it can be kept for up to around two to three months. Defrost individual portions at room temperature.