Oven to 140 C. You’ll need a 24cm tart tin with removable base. Toast the hazelnuts in the oven for 5-10 minutes until they are lightly toasted. Let them cool down a bit and whizz them in the food processor along with the muscovado sugar, flour and butter. Whizz until it forms a breadcrumb-like texture and then press down the crumbs into the tart tin, as you would a biscuitey cheesecake base. Spread evenly, which is sometimes easier to do with a wet spatula, and then freeze or chill while you get the chocolate filling sorted.
Melt the dark chocolate in a bowl over simmering water. While that is melting, bring the cream just up to the boil. Whisk the egg yolks and sugar until pale, thick and doubled in volume. Pour the hot cream onto the melted chocolate and whisk. This will look scarily seized up for a second or two but keep whisking. Then fold the melted chocolate into the whisked egg yolks. It should become very thick and glossy as it’s mixed in and starts to cool down slightly. Put the tart tin onto a flat baking sheet, which will be easier to transport. Pour the chocolate filling in and bake for about 45 minutes. There will be a slight wobble in the middle, but this is gorgeous after it’s cooled right down and served cold with a big blob of cream to which you’ve softly whipped in some vanilla extract and icing sugar.