Melt the chocolate mixture:
In a large heatproof bowl, combine the dark chocolate, unsalted butter, and golden syrup. Place the bowl over a pot of simmering water (ensure the bottom of the bowl does not touch the water). Stir occasionally until the mixture is fully melted and smooth.
Prepare the mix-ins:
In a separate large mixing bowl, combine the crushed digestive biscuits, mini marshmallows, roasted nuts, and dried cranberries (if using). Stir to distribute evenly.
Combine and mix:
Pour the melted chocolate mixture over the dry ingredients. Using a spatula, gently fold the ingredients together until everything is well coated with the chocolate mixture.
Transfer to pan:
Line a 20cm x 20cm (8-inch x 8-inch) square baking tin with parchment paper, allowing the edges to overhang for easy removal. Pour the rocky road mixture into the prepared tin, pressing down lightly to ensure an even layer.
Chill:
Sprinkle the top with desiccated coconut (if using). Place the tin in the refrigerator and allow it to chill for at least 2 hours or until set.
Cut and serve:
Once set, remove the rocky road from the tin using the parchment paper overhang. Place on a cutting board and slice into 16 squares. Serve immediately or store in an airtight container in the refrigerator.