In your Kenwood mixer, using the paddle attachment, crumb together all the ingredients except the eggs on a medium to low speed. The crumb should be like fine sand.
Add the whole eggs in two stages by gently combining on low speed and making sure not to over-work the dough at this stage.
Remove the dough from the machine and combine by hand, the dough can be separated into two balls and cling filmed and placed into the fridge to rest and chill Remove the dough from the fridge and allow to soften slightly before rolling. Lightly dust your worktop and using a rolling pin roll the dough out to a ½ cm thick round disc, being careful not to overheat the dough when working with it as it will break when lining the case.
Using a fluted 12-inch tart case lay the rolled pasty into the case and start to work the edges into the tin. Leave the excess pastry overhanging the tin, as this can be trimmed after baking. TIP: When lining pastry, use a small soft ball of pastry dipped in flour to help push the edges into the tin, this is more delicate then using your fingers.
Place your lined tin into the fridge for 30 minutes to rest, and preheat your oven to 180c.
To bake the tart, take a large sheet off baking paper and crumple it up into a ball. Open the paper up inside the case and fill it with baking beans or rice, this will stop the pasty rising and give you a flat tart.
Bake for 15 minutes and then remove the paper and beans and cook for a further 10 minutes until the pastry is cooked through and crisp
One the pastry is cooked, remove it from the oven and brush the egg yolk inside the hot tart case, this will seal any little holes in the pastry and make it watertight.
Allow the case to cool before adding the filling
For the chocolate tart mix:
In a saucepan, bring the milk, cream and zest to a light boil.
Add the chocolate to a large bowl and pour over the hot cream mixture and mix well with a hand mixer or whisk, make sure not to over-mix, which will incorporate air into the mix.
Allow the mix to cool.
Add the eggs to the chocolate mixture and mix well again
Pass the mixture through a fine sieve, ready to add to the tart case
Preheat the oven to 85c pour in the chocolate tart mix filling to the very top of the tart and place the tart case into the oven.
Cook the tart until the tart mix is just set (approx. 25 minutes)
Remove the tart and allow it to cool to room temperature then, using a sharp knife, trim the excess pastry away from the tart case.
Remove the tart from the case and portion it into your desired size portion
Finish each slice with a dusting of cocoa powder, orange zest and a pinch of sea salt