Line a shallow dish of about 10 x 8 inches (25 x 20 cm) with parchment paper or aluminum foil. Leave an overhang on each side, so you will be able to lift the fridge cake easily out of the dish later on.

Roughly crush the digestive biscuits or graham crackers into small pieces.

Put the butter in a medium pan over low heat. As soon as it starts to melt, add the honey and both chocolates. Keep over the heat until you can see the chocolate melting into a puddle, then turn off the range, and leave for about 5 minutes. Everything should melt smoothly in the residual heat. Stir gently to mix.

Add the digestive biscuits or graham cracker pieces, the hazelnuts, and a pinch of flaky sea salt. Stir until everything is well-coated in the chocolate.

Tip into the prepared dish, and use a potato masher to tamp it down, until roughly flattened. Cover, and put into the fridge for at least 2 hours, or until solid.

Turn out carefully, peel off the parchment paper, and cut into squares or long slices. Eat quickly, cold from the fridge, before it melts.