Line a 20cm square baking tin with cling film.
Place the butter, golden syrup, and broken milk and dark chocolate pieces into a large microwaveable bowl. Microwave in 30-second bursts (stirring in between) until everything is melted together.
200 g Dark Chocolate, 100 g Milk Chocolate, 100 g Butter, 150 g Golden Syrup
Fold in the crushed shortbread biscuits, cocoa powder, dried cranberries, white chocolate chips, and orange zest, and mix until all the ingredients are coated in the chocolate mixture.
250 g Shortbread Biscuits, 2 teaspoon Cocoa Powder, 75 g Dried Cranberries, 100 g White Chocolate Chips, ½ Orange
Press the tiffin firmly into a lined tin (use the back of a heavy metal spoon to really compact it down).
For the milk chocolate topping, melt the milk chocolate pieces (and a few drops of vegetable oil) in the microwave in 30-second bursts to ensure it doesn't go grainy. Spread the melted milk chocolate onto the tiffin base, sprinkle over the Christmas sprinkles, and refrigerate for at least 2 hours, but preferably overnight
200 g Milk Chocolate, ¼ teaspoon Vegetable Oil, 4 tablespoon Christmas Sprinkles
Take the tiffin out of the fridge 10-15 minutes prior to cutting it, to ensure the chocolate topping doesn't crack.