First, make the tart case. Melt the chocolate and butter over a bain-marie, or in short bursts in the microwave. In a large bowl, mix the hazelnuts, bran flakes and salt, then pour in the melted chocolate mixture and stir well.
Press into the bottom and up the sides of a 20cm tart shell; it doesn’t matter if it doesn’t reach the very top – you just want a good edge of roughly 2cm. Put in the fridge to chill.
Now make the miso caramel. Set a saucepan over a medium heat, sprinkle in the sugar and leave it to turn into a dark caramel. Add the butter, whisk well, pour in the cream, then let it bubble for a minute before removing from the heat and whisking in the miso.
Set aside and, once it has cooled slightly, pour into a heatproof dish and leave to cool; you want it to be at pouring consistency, but not hot.
Now make the ganache. Heat the cream in the same pan until steaming. Put both chocolates in a large bowl, pour over the hot cream, set aside for a minute, then stir with a whisk, from the middle outwards, until melted and smooth. Pour into the cooled tart shell and set aside again for two minutes.
Drizzle the caramel thickly all over the ganache. Use a butter knife to drag the caramel through the ganache, then put in the fridge to set. The tart will keep for two to three days, but is best eaten as soon as the ganache has set.