Line an 18cm x 18cm square tin with baking parchment. Roughly chop both bars of chocolate and put in a heatproof bowl with the butter. Set the bowl over a pan of gently simmering water (ensuring the bottom of the bowl doesn’t touch the water).
Stir occasionally until everything has melted. Take off the heat; stir until combined.
Stir most of the dates, pistachios, pumpkin seeds and ginger nuts into the chocolate mixture, reserving a few to decorate the top. Add a good pinch of salt, then tip into the lined tin.
Press down with the back of a spoon to smooth the surface. Scatter over the remaining dates, nuts, seeds and biscuit chunks.
Leave at room temperature to set for 1 hour, then cover and chill until set firm. Take out of the fridge for 5 minutes before cutting into slices about 2cm thick and then in half to serve. Store any leftover cake in an airtight container in the fridge for 1 week.