Preheat the oven to 220 C / Gas 7. Lightly grease a tart tin.
Mix flour, butter and egg by hand, add a little water and cocoa and knead until mixture is smooth. Add more cold water as necessary, only to get the mixture to come together. Form a ball, place in a bowl with a clean cloth and let the pastry rest in a cool place at least 30 minutes.
Roll out the pastry on a clean work surface dusted with flour, and transfer to the prepared tart tin. Prick the bottom of pastry with a fork, cover with foil and rice or beans, then bake in the oven for 20 minutes. Cool completely.
For the filling, heat the cream in a saucepan over medium heat. Just before it comes to the boil, remove from heat, add white chocolate and close the lid. Let stand 5 minutes, then stir with a wooden spoon until smooth. Pour over the cooled chocolate pastry shell and allow to cool to room temperature. Keep at least 2 hours in the fridge until the chocolate filling sets.
Top with fresh berries, jam, passion fruit or a drizzle of dark chocolate.