Line your tray-bake tin with greaseproof paper. A 23x23cm baking tray (or equivalent) is ideal. Set aside.
Crush your digestive biscuits. You want most of it to be quite fine but leave some bits in bigger chunks to give your Tiffin a bit of texture.
Add the butter, sugar, cocoa powder, golden syrup and 30g of milk chocolate to a large bowl and melt over a bain-marie or in short burst in the microwave, stirring often.
Stir in the biscuits and raisins and mix well. Press the mixture into your pre-lined tin.
Melt the remaining milk chocolate in a bain-marie or in short bursts in the microwave, stirring often. Pour over the biscuit mixture and smooth over evenly. Decorate with melted white chocolate if desired.
Leave to set in the fridge for at least 2 hours but ideally over night before cutting into individual portions.