Place the butter, chocolate and golden syrup into a heavy-based saucepan. Heat very gently, stirring frequently, until everything has melted. Take care not to overheat this - it should only be warm, not hot. I tend to remove the pan from the heated before everything is melted and let the residual heat take care of the rest.
Place the ginger nuts into a large freezer bag and bash them with a rolling pin until you have a chunky rubble - you don't want the pieces to be too small.
Stir the biscuits into the chocolate, followed by the mini marshmallows, dried cranberries and grated orange zest. Mix well.
Have your 6" tins ready. Scoop out about a cup of rocky road mixture and spread out in the base of the tin. Repeat for the other tin so that you have two discs in total. Spread out the remaining mixture on a lined baking sheet. Refrigerate until firm - at least a couple of hours.
Melt the white chocolate. Drizzle over the sheet of rocky road (not the round pieces) and refrigerate until set.