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Tear the pastry into a food processor, add the cocoa, pulse until combined, then knead a few times on a floured surface until evenly brown. Use to line a 23cm fluted tart tin, and prick the base several times with a fork. Chill or freeze until very firm. Heat oven to 200C/180C fan/gas 6.
Put the tart tin on a baking sheet then line the pastry case with overhanging baking parchment. Fill with baking beans, then bake for 15 mins, or until the sides of the pastry have set firm. Remove the beans and parchment, then bake for 10-15 mins more until the pastry feels dry all over. Leave to cool.
Strain the orange juice, then measure 250ml into a heavy-based saucepan. Add the cream and bring to the boil. Strain again if necessary. Whisk the eggs and sugar in a bowl, then gradually whisk the hot cream into the mix. Wipe out the pan, return the custard to it and cook gently for about 10 mins over a low heat, whisking frequently until steaming and thick enough to coat the back of a spoon. Strain into a jug, whisk in the zest and pour the custard into the pastry case. Cool, then chill for at least 4 hrs, or ideally overnight, until set.
Melt the drizzle ingredients together until smooth, spoon some over the tart, then return the tart to the fridge. Serve the rest of the drizzle warm for spooning alongside the chilled tart.