To make the pastry, put the plain flour, cocoa powder and caster sugar in a bowl and stir together
Add the butter in chunks and use your fingers to rub the butter and flour mixture together until it resembles bread crumbs. You can also do this with a fork, a pastry blender or in a food processor
Add the milk little by little (you may not need it all, or you may need a little bit more) and form into a dough. Don't be tempted to add extra liquid
Wrap it in cling film and chill it in the fridge for 30 minutes
Pre-heat your oven to 180C Fan/200C/350F/Gas Mark 5
Using a light dusting of flour on your surface and rolling pin, roll the pastry out to fit a 9" tart tin
Smooth the pastry out in the tin, trim the edges and prick the bottom with a fork
Scrunch up some baking paper and cover the pastry, then add baking beans (you can also use dried beans or rice). Bake it for 15 minutes
Remove the pastry from the oven and remove the baking paper and baking beans. Put it back in the oven and bake it for 10 minutes
Take the pastry out of the oven, reduce the temperature to 160C Fan/180C/350F/Gas Mark 4
To make the custard filling warm the double cream in a pan on a low-medium heat
Once it starts steaming, take it off the heat and add the dark chocolate. Leave it for a few minutes, then stir until smooth and all of the chocolate is melted
Whisk the eggs, caster sugar and orange extract together in a mixing bowl
Slowly pour the chocolate and cream mixture into the egg and sugar mixture, whisking constantly as you pour. Once mixed, transfer to a jug
Put the pastry case on the oven shelf first, then pour the filling in using the jug. This will stop the filling from spilling over the edges if you carry it to the oven
Bake for 30 minutes, then leave to cool completely before decorating. The filling will puff up, but will sink down as it cools
To decorate, whip up the double cream with the vanilla extract and icing sugar using an electric whisk. Pipe onto the tart, or dollop on with a spoon
Decorate with some Terry's Chocolate Orange slices if you like
Serve immediately, store leftovers in the fridge and eat within 2 days