Sift the flour, icing sugar, cocoa and ½ tsp salt into a bowl. Rub the butter in with your fingers until the mixture resembles breadcrumbs. Add the egg yolks along with 2 tbsp ice-cold water, and continue to mix with your hands until a soft dough forms. Alternatively, pulse the ingredients together in a food processor. Wrap the dough and chill for at least 1 hr. Will keep chilled for up to two days.
Line the base of a 23cm loose-bottomed tart tin with baking parchment. Roll the dough out on a lightly floured surface to the thickness of a £1 coin, then use it to line the tart tin, pressing it up the side and leaving some overhanging. Cover and chill for another 1 hr.
Heat the oven to 190C/170C fan/gas 5. Line the pastry with baking parchment and fill with baking beans. Bake for 20 mins. Remove the beans and parchment and bake for 20 mins more until crisp. Leave to cool completely, then use a sharp knife to trim the excess pastry.
To make the curd, put the passion fruit pulp in a food processor and blitz to separate the seeds and flesh. Push through a sieve over a medium saucepan, then discard the seeds. Tip in the remaining ingredients and set the pan over a low heat. Whisk until the butter has melted, then, using a wooden spoon, stir until the curd has thickened, about 10 mins. (Don’t turn up the heat to speed up the process, as the eggs will curdle.) Stir well, especially at the edge, as this is where the curd can catch. Sieve into a bowl, leave to cool, then spoon into the case. Chill the filled tart for 30 mins.
To make the ganache, melt the chocolate, cream, sugar and a pinch of sea salt in a heatproof bowl over a pan of simmering water, making sure the bowl doesn’t touch the water, stirring until glossy and thick. Remove from the heat, leave to cool for 15 mins, then pour over the curd layer. Scatter over the cocoa nibs, if using, then chill for at least 4 hrs, or overnight. Dust with cocoa, then slice and serve with crème fraîche.