Pre-heat the oven to 175º/350ª degrees.
Sift the chestnut flour, hazelnut and flaxseed meals and salt into a bowl. Add the melted coconut oil and the water, and work the mixture with your hands until you get a firm dough. Divide the dough into 4equal portions.
Using a pastry brush, grease your tart tins with extra melted coconut oil. Press one piece of dough into one of the tart shells and, working with your hands, make it evenly cover its bottom and sides. Using a fork, pinch the dough a few times. Repeat this procedure with the remaining dough and shells.
Cover the pastry shells with greaseproof paper, fill it with baking beans or pie weights and bake blind for 15 minutes. After that time, remove the beans and paper and bake the tarts for additional 10 minutes, or until they’re golden brown.
In a small pan, heat the coconut milk with the ground cardamom until it almost starts to boil (don’t let it boil, though). Remove from the heat and add the chocolate. Stir around with a wooden spoon until the chocolate has fully melted. Let the mixture cool a bit, but not to the point of starting to set.
Divide the mixture evenly between the four tart shells. Let it set for 1 to 2 hours at room temperature. After that time, you can refrigerate the tarts for 1 additional hour if you prefer them cold (I actually ended up eating a forth of one at room temperature and enjoyed it). Unmold before serving and sprinkle with cocoa powder and flaky sea salt or fleur de sel.