• Line a 20cm square baking tin with clingfilm or nonstick baking paper, making sure it overhangs the edges of the tin.
• Place the chopped dark and milk chocolate in a heatproof bowl, along with the butter and golden syrup. Microwave in 30-second bursts, stirring with a wooden spoon between each burst until smooth. Let the mixture cool to room temperature.
  150 g Dark Chocolate, 150 g Milk Chocolate, 100 g Butter, 125 g Golden Syrup
• In a large bowl, combine the crushed digestives and Maltesers, mixing them together evenly.
  150 g Maltesers, 250 g Digestives
• Pour the cooled chocolate mixture over the crushed biscuits and Maltesers. Stir well until everything is fully coated.
• Transfer the mixture into the tin. Press it down firmly using the back of a spoon to create an even layer. Place the tin in the fridge while you prepare the topping.

For the topping

• Melt the dark chocolate and white chocolate separately in the microwave, using short bursts. Stir in a few drops of vegetable oil into each bowl to prevent the topping from cracking when cut.
  100 g Dark Chocolate, 100 g White Chocolate, ¼ teaspoon Vegetable Oil
• Spoon dollops of both melted chocolates over the chilled tiffin. Use a skewer or toothpick to swirl the chocolates together, creating a marble effect.
• Scatter over the Maltesers reserved for the topping and press down lightly.
  150 g Maltesers
• Refrigerate for at least 2 hours. Once set, lift the tiffin out of the tin, place it on a chopping board, and cut into bars with a sharp knife.