Put the butter, golden syrup and 500g white chocolate in a heatproof bowl, and melt them in a microwave – or by setting the bowl over a pan of simmering water – until you have a smooth mixture. Pour this over the biscuit mix and stir until everything is coated. Spoon the mixture into the tin and press down. Melt the rest of the chocolate and spread it on top, then shake the tin to settle the surface. Top with more chopped ginger if you like. Leave to cool and set completely (put it in the fridge if you need to). Lift the tiffin out of the tin, cut into squares and gift wrap.