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.
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.
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.
You can make this tart a day in advance. Simply make as instructed, cover tightly, and keep in the fridge overnight ready for the next day.
Leftovers will keep fresh in the fridge for up to 3 days.
The calories are an estimate only.