For the tart dough
Place the butter and sugar in the food processor mixing bowl and combine. Add the eggs and continue mixing for 2 minutes more. Replace food processor component with the kneading fixture and add remaining ingredients until they form a homogenous dough. Cover dough with a towel and allow to “rest” for 40 minutes in the fridge. Then, place between 2 sheets of parchment paper and roll out with a rolling pin, starting from the centre of the dough and pressing consecutively in all directions (up, down, right and left), to achieve a round and uniform dough approx. ½ cm. thick (it must be larger than the tart dish). To prepare the tart dish, grease with a little melted butter and dust with cocoa. Shake well to remove excess cocoa. Lay dough in the dish, pierce with a fork and cover its surface with parchment paper and some beans (to avoid the dough rising during baking). Bake at 170°C for 25 minutes. Take out of the oven, remove beans and parchment paper and allow to cool.
For the chocolate filling
Soak the gelatin in water until soft and malleable. In the food processor mixing bowl beat the eggs and the sugar until fluffy. Place cream, milk and salt in a pot and heat to just before boiling point. Carefully combine eggs and cream and milk mixture by slowly pouring the milk mixture into the egg mixture stirring constantly to avoid cooking the egg. Return mixture to the pot and cook over low heat stirring constantly until the mixture begins to thicken. Remove from the heat, add gelatin, chocolate, vanilla, zest and combine well. Strain through a fine sieve, allow to cool to approx. 30° and fill tart. Place in the fridge until set.
For the chocolate sauce
Heat water and sugar in a pot until boiling point. Slowly add cocoa and cream. Simmer for approx. 30 minutes until the sauce thickens, stirring with a spatula. Strain and cool in a cold bain-marie.