Line a 20cm x 20 cm (8 inch x 8 inch) square brownie pan. You can use a longer, rectangular pan but the bars will be thinner.
In a pan (or large microwavable bowl) add the butter, golden syrup, cocoa powder and salt and melt.
Add the raisins and mix well.
Whizz the biscuits in a food processor or crush in a bowl with the end of a rolling pin, then add to the chocolate and raisin mixture.
Pour into the lined brownie pan and press down evenly until well packed, then pop in the fridge to set.
Once set melt the chocolate. You can melt it in a bain marie (see notes below) or in a bowl in the microwave (keep an eye on it, it doesn't take long).
Pour the melted chocolate over the tiffin and smooth out in an even layer and pop back in the fridge,
When the chocolate is partially set, score the bars with a knife, then allow to continue setting in the fridge. When set, cut into bars.
Store in an airtight container in the fridge.
Enjoy!
Notes
To melt the chocolate in a bain-marie, pop a bowl over a pan of simmering water. The bowl should not touch the water or the chocolate will seize. Add the chocolate to the bowl and stir until melted.
These bars can be kept in the fridge for up to 5 days in an airtight container.
You can cut these bars into smaller pieces to cut the calories. If you cut the bars in 5 rows in each direction (25 bars), they will be 145 calories per bar.
You can also freeze these bars. Wrap the whole tiffin or individual bars in freezer-friendly baking paper and store in a freezer bag in the freezer for up to 2 months. Take one out when you need a treat. They don't take long to defrost in the fridge.