Line a square cake pan with parchment paper. I used a 20cm (8-inch) pan. A wider pan will make thinner slices of tiffin.
Put the digestive biscuits (or Graham Crackers) in a Ziploc bag and bash them with a rolling pin until they are mostly crushed with a few big chunks remaining. Yes you could. pulse them in a food processor, but this is the Scottish way to do it!
Melt the chocolate, butter, golden syrup, sugar, cocoa powder, and salt together, either over a double boiler or in 30-second bursts in the microwave - stirring in between until everything is completely melted and smooth.
Tip the crushed biscuits into the chocolate mixture and stir until evenly distributed.
Spoon the tiffin mixture into the prepared pan and spread it around, pressing down slightly to make an even layer.
Melt the chocolate for the topping in short bursts in the microwave, then stir in the oil.
Pour over the top of the tiffin and smooth it out. You can shake or tap the pan to encourage the chocolate to spread out evenly.
Put in the fridge to set for at least 2 hours (even better if you can leave it overnight).
Once chilled, let the tiffin sit at room temperature for a few minutes to soften up, then use a sharp knife to cut it into 16 squares.
Notes
Raisins - Scottish Tiffin traditionally has raisins in it, but I personally think raisins in baked goods are a crime, so I leave them out. If you'd like to add them, just stir in 150g (1 cup) of raisins at the same time as the biscuit crumbs.