01 - Preheat your oven to 350°F. Lightly grease a 9-inch tart pan with a removable bottom, preparing it for the crust.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, powdered sugar, and salt until well combined. Add the cold, cubed unsalted butter and cut it into the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs.
03 - Incorporate the large egg yolk and 2 tablespoons of cold water into the crumb mixture. Mix just until the dough comes together, adding the extra tablespoon of water only if needed to achieve a cohesive dough.
04 - Evenly press the prepared dough into the bottom and up the sides of the tart pan. Prick the entire base thoroughly with a fork to prevent bubbling during baking. Chill the formed crust in the refrigerator for 15 minutes.
05 - Bake the chilled tart crust for 15 minutes, or until it appears set and slightly firm to the touch. Once baked, remove from oven and allow it to cool completely on a wire rack before proceeding.
06 - Place the finely chopped bittersweet or semisweet chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer, being careful not to bring it to a rolling boil. Pour the hot cream over the chopped chocolate, let it sit undisturbed for 2 minutes to soften the chocolate, then stir gently until the chocolate is completely melted and the mixture is smooth.
07 - Stir in the softened unsalted butter and pure vanilla extract into the chocolate mixture until the ganache is thoroughly combined, glossy, and uniform in texture.
08 - Pour the prepared ganache into the cooled tart shell, gently smoothing the top surface with a spatula if necessary. Refrigerate the tart for at least 2 hours, or until the ganache is completely set and firm.
09 - Just before serving, elegantly scatter the pomegranate arils evenly over the set ganache. If desired, drizzle with a touch of honey or pomegranate molasses and sprinkle lightly with flaky sea salt for enhanced flavor and presentation. Slice and serve the tart chilled.