Start with the biscuit base: put the Hobnobs into a plastic bag and bash them into crumbs with a rolling pin (or if you’re feeling less destructive, give them a quick blitz in a food processor).
Stir in the cocoa, sugar and salt, followed by the butter. Line the base of a 25cm round tart tin with greaseproof paper, grease the sides with some extra butter, then firmly press the crumb mixture into the tin, taking extra care with the sides and aiming for an equal thickness throughout. Pop it into the freezer to chill for 10 minutes, then take out, line with greaseproof paper and fill with baking beads. Bake at 200C/390F/gas 6 for 10-12 minutes. Take out of the oven and set aside to cool.
Meanwhile, make the chocolate fudge filling. Put all the ingredients (apart from the salt) into a saucepan and gently warm over a low-medium heat. Keep stirring the mixture until the chocolate has melted and the ganache is smooth and glossy. Pour into the cooled biscuit base and chill in the fridge for a couple of hours.
Sprinkle the sea salt over just before serving.