Combine flour, cocoa powder, salt, and sugar in a food processor. Add the cold, cubed butter and pulse until the mixture looks like coarse crumbs. Add the egg and pulse until the dough just begins to clump together. If it feels too dry, add a tablespoon of ice water at a time, pulsing until it forms a cohesive ball without overmixing.
Gently shape the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes to firm up.
Preheat your oven to 350°F (175°C). On a lightly floured surface, roll out the chilled dough to fit a 9-inch (23cm) tart pan. Carefully place it into the pan, pressing gently into the edges. Trim any excess dough and prick the bottom all over with a fork. Line the crust with parchment paper, fill with pie weights (or dry beans/rice), and bake for 15-20 minutes. Remove the weights and paper, then bake for another 10-15 minutes until the crust looks dry and set. Let cool.
Cut the kataifi into small, quarter-inch pieces. Melt two tablespoons of butter in a medium frying pan over medium heat. Add the kataifi and toast, stirring constantly, until it turns beautifully golden brown and crispy. Transfer the toasted kataifi to a bowl, then stir in the natural pistachio paste and tahini until well combined. If you prefer a sweeter filling, mix in your optional natural sweetener now.
Spoon the pistachio-kataifi filling evenly into the cooled, pre-baked tart shell. Gently press it down and then refrigerate the tart for 30 minutes to allow the filling to set.
Place your chopped dark chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer around the edges. Immediately pour the hot cream over the chopped chocolate. Let it sit undisturbed for one minute, then gently stir until you have a smooth, glossy ganache.
Carefully pour the rich ganache over the chilled pistachio filling, spreading it evenly with a spatula. Return the tart to the refrigerator for at least 1 hour, or until the ganache is fully set.
Before serving, generously sprinkle chopped pistachios over the top of the tart for an extra crunch and a beautiful finish. Slice and enjoy your delightful summer treat!