01 - Preheat oven to 350°F. Lightly grease a 9-inch tart pan with a removable bottom.
02 - In a mixing bowl, whisk together all-purpose flour, cocoa powder, powdered sugar, and salt. Add the cold, cubed butter and rub it into the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs.
03 - Incorporate the egg yolk and 2 tablespoons of cold water into the crumb mixture. Mix until the dough just comes together, adding the extra tablespoon of water if needed to achieve a cohesive dough.
04 - Press the prepared dough evenly into the bottom and up the sides of the greased tart pan. Prick the base of the dough liberally with a fork. Chill the dough in the refrigerator for 15 minutes.
05 - Bake the chilled tart crust for 15 minutes, or until it is set and slightly firm. Allow the baked crust to cool completely on a wire rack before proceeding.
06 - For the ganache, place the finely chopped chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer (do not allow it to boil). Pour the hot cream over the chopped chocolate, let it sit undisturbed for 2 minutes, then stir thoroughly until the chocolate is fully melted and the mixture is smooth.
07 - Stir in the softened butter and pure vanilla extract into the chocolate mixture until the ganache is glossy and well combined.
08 - Pour the finished ganache into the cooled tart shell, spreading it evenly to smooth the top surface. Refrigerate the tart for at least 2 hours, or until the ganache is completely set.
09 - Just before serving, scatter the pomegranate arils evenly over the set ganache. If desired, drizzle with honey or pomegranate molasses and sprinkle with flaky sea salt.
10 - Slice the chilled tart into portions and serve for optimal enjoyment.