Over a gentle heat, melt butter, sugar, milk and golden syrup in a pan.
Whilst butter is melting, break up the biscuits into small crumbs. Add half to the melted butter, along with the drinking chocolate and cocoa powder (OK to use just one of these if that's what you have in the cupboard). Once mixed well, add rest of digestives. Add in the raisins (and cherries/nuts if using) and mix through.
Tip into a greased tray and press down well. It should ideally be around 2cm thick. Leave in fridge to set for 1 to 2 hours.
Melt the chocolate, pour over the biscuit base and return to the fridge for another 30 minutes to 1 hour to set, before cutting into squares.