In a small bowl or jug, whisk together the egg yolk, water, and orange juice.

In a large bowl, mix together the flour, zest, cocoa powder, sugar, and salt.

Add the butter, and using a pastry cutter, or two forks, cut the butter into the flour mixture until it resembles coarse meal with pea-sized pieces.

Add the egg mixture, and stir with a rubber spatula or wooden spoon until the dough starts to come together.

Transfer the dough to a floured surface, and gather into a ball. Shape the dough into a 1-inch thick disc.

Wrap tightly in clingfilm, and refrigerate for at least an hour (and up to 5 days). You can also freeze for up to 1 month. Thaw overnight in the fridge before use.

On a lightly floured surface, roll out the dough to line an un-greased 23cm/9-inch fluted tart tin with a loose base. Press lightly into the tin, making sure not to stretch the dough.

Roll the pin over the top of the tin to cut off the excess dough. Prick the base all over with a fork. Place in the freezer for 30 minutes.

Preheat the oven to 180C/350F/Gas 4.

Line the chilled pastry crust loosely with foil or parchment paper, and fill with baking beans. Bake for 15 minutes, then remove the beans and foil and bake for a further 4 minutes.

Brush the base and sides of the crust with beaten egg, and place back into the oven for 1 last minute. Leave to cool on a wire rack.

In a microwaveable bowl, melt the chocolate in 20 second intervals, stirring after each one. Leave to cool for 5 minutes. In a separate bowl, mix together the cream, sugar, eggs and orange juice until combined.

Gradually stir in the chocolate, then pour the mixture into the pie crust. Bake for 25 - 30 minutes, or until the top is set and the centre is no longer wobbly.

Serve warm or cold.