Line a 20cm square baking tin with non stick baking paper or clingfilm.
Place the butter, golden syrup, and broken milk and dark chocolate pieces in 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 digestive biscuits and cocoa powder until all of the biscuits are coated in the chocolate mixture.
250 g Digestives, 2 teaspoon Cocoa Powder
Press the mixture 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 and refrigerate for at least 2 hours, but preferably overnight.
200 g Milk Chocolate, ¼ teaspoon Vegetable Oil
Take the tiffin out of the fridge 10-15 minutes prior to cutting it into bars, to ensure the chocolate topping doesn't crack.