Preheat the oven to 170ºC / 150ºC fan / gas mark 3. 

Place the chocolate in a tall jug. 

Add the egg yolks and three quarters of the brown sugar to the bowl of the stand mixer and whisk until it turns fluffy and pale, about 3-4 mins.  

Add the milk, cream, remaining brown sugar and vanilla to a saucepan over medium heat until steaming, about 2-3 mins. Take it off the heat and pour the mixture over the chocolate in the jug. Leave to stand for 1 minute then emulsify with a stick blender.  

Gradually add the chocolate mixture to the egg mixture while the whisk is running, until mixed really well.   

Divide the mixture between 4 ramekins. Place the ramekins in a high sided baking tray and carefully pour enough boiled water around them so that the water level comes to halfway up the ramekins. Make sure not to splash water into the ramekins as you do this. Bake for 30 to 40 minutes, until there is a gentle wobble in the middle. Leave to cool, then chill for at least 1 hour or overnight.  

To finish, sprinkle each of the baked custards with the caster sugar and caramelise using a blowtorch, or you can do this under the grill. To do this in the oven, preheat the grill to its highest setting then place the oven rack as high as possible. Bake for a few minutes, until the sugar is caramelised.  

Pour the cream into a stand mixer bowl and refrigerate it until ready to whip (or chill the bowl in advance). Whip until it starts to thicken, then add the sugar and vanilla and continue to whip until it reaches medium firm peaks.  

Fit a piping bag with a nozzle of your choice, then fill with the chantilly cream. Pipe this onto the creme brulee. Roughly chop the pistachios, then scatter these over to serve.