First make the dough for the base. Combine the flour, cocoa powder sugar and salt, if using, in a large bowl. Stir well.
Using your fingertips, rub the cold butter cubes into the flour till the mixture resembles coarse breadcrumbs with a few larger bits of butter scattered throughout. These bits are what create a flaky dough.
Add 1 tbsp of ice water and bring the dough together. If you can clump it between your fingers, it does not need any more water. Add the second tablespoon of water only if absolutely needed.
Form the dough into a rough ball, place on a large piece of clingfilm and pat into a 3/4th inch thick disc. Wrap tightly and refrigerate for 1 hour.
In the meantime, combine the cream, butter, rosemary and orange zest in a small saucepan. Place on low heat and bring to a simmer, stirring constantly to ensure the butter melts and doesn't stick to the pan. Cover and set aside to infuse at room temperature while you make the tart base.
At the end of 1 hour, lightly grease an 8" tart tin or pie dish. If you have one with a removable bottom, that makes life easier.
Unwrap the cold dough and place on a lightly floured silicone baking mat or on the kitchen counter.
Roll using a lightly floured rolling pin, to an approximately 9" circle, sprinkling more flour to prevent any sticking.
Gently lift the dough and place in the tart tin, patching up any torn bits with scraps of dough. Trim the edges and place the tin in the fridge for 30 minutes.
Preheat the oven to 180 C. Take the tart tin out of the fridge and prick the base all over with a fork to prevent it from puffing up too much. Bake for 15 to 20 minutes until lightly browned and the sides are slightly puffed. Set aside to cool. Leave the oven on.
Now return to the filling. Place the chopped chocolate in a bowl. Bring the infused cream back to a simmer and pour over the chocolate. Stir and set aside for 5 minutes. Then stir again till smooth and combined. If it doesn't look well combined, set the bowl over a pan of simmering water and stir till the chocolate is melted through.
In a mixing bowl,beat the sugar, eggs and vanilla for about 4 minutes until light and thick. Very gently fold in the chocolate cream mixture, taking care not to deflate the eggs as far as possible.
Pour this filling into the baked tart shell and bake again for 15 to 20 minutes until the top is set and a toothpick comes out with just a few moist crumbs, much like a flourless cake.
Set aside to cool at room temperature for about an hour, then dust with cocoa powder, decorate with more rosemary leaves and orange zest and slice and serve! The tart should be kept refrigerated but tastes great cold too :) It will keep in the fridge for about a week. Enjoy!