1. Preheat the oven to 110ºC, gas mark ¼. Line 3 baking sheets with parchment and draw a 23cm circle on each. For the meringues, tip the pistachios onto a small baking tray and the caster sugar onto another. Bake the nuts for 4 minutes, until lightly toasted, and the sugar for 5 minutes, until hot to the touch. Finely chop the nuts; cool.
2. Tip the egg whites and a pinch of salt into a freestanding food mixer with a whisk attachment; whisk to almost stiff peaks. Tip in the hot sugar, whisk for 1 minute, then scrape down the sides. Whisk on high for 3 minutes, until stiff, glossy and the sugar has dissolved.
3. Gradually add the muscovado, a spoonful at a time, whisking between additions and scraping down the sides. Whisk on full speed for 2 more minutes, until the sugar is incorporated and the meringue is cool, silky and smooth. (Mix for another minute if not.)
4. Divide the meringue between the baking sheets and spread to the edges of the 23cm circles using a palette knife. Sprinkle the chopped nuts on top. Bake, swapping the trays from top to bottom halfway through cooking, for 1 hour-1 hour 30 minutes, until crisp. Turn the oven off; leave the meringues inside for 45 minutes to cool.
5. For the filling, chop the chocolate and melt, stirring until smooth; leave to cool slightly. Put the egg yolks in a heatproof bowl and set aside. Warm the sugar and 3 tbsp water in a pan over a low heat to dissolve. Bring to the boil and simmer for 30 seconds, then pour over the yolks, whisking constantly with a balloon whisk, until smooth. Add a pinch of salt, set over a pan of simmering water (don’t let the base touch the water) and whisk for 2-3 minutes, until pale and thick.
6. Remove from the heat and whisk for 2-3 minutes, until cool and the consistency of softly whipped cream. Fold in the cacao powder, then the melted chocolate; stir until smooth. Stir in 150ml double cream.
7. Whip the remaining 500ml cream to soft peaks. Put a meringue disc on a plate and spread with ½ the chocolate cream and ½ the whipped cream. Repeat and top with the fi nal meringue. Dust with icing sugar and scatter over candied chocolate pistachios, if liked.