Step 1: Prepare the Tart Crust: In a large bowl, whisk together flour, powdered sugar, and salt. Add cold, cubed butter and cut it in with a pastry blender or your fingertips until the mixture resembles coarse meal with some pea-sized pieces of butter remaining. In a small bowl, whisk egg yolk with 1 tablespoon of ice water. Add to the flour mixture and mix until just combined, adding more ice water, a teaspoon at a time, if needed, until the dough comes together. Do not overmix. Form into a disc, wrap in plastic, and chill for at least 30 minutes.
Step 2: Blind Bake the Crust: Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough to an 11-inch circle. Carefully transfer to a 9-inch tart pan with a removable bottom. Press gently into the pan, trim excess dough, and prick the bottom all over with a fork. Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove paper and weights, then bake for another 5-7 minutes, or until lightly golden. Let cool completely on a wire rack.
Step 3: Make the Black Sesame Paste: If using raw seeds, lightly toast 1/2 cup black sesame seeds in a dry pan over medium-low heat for 3-5 minutes, until fragrant. Be careful not to burn. Let cool. In a high-speed blender or food processor, combine toasted black sesame seeds and 2 tablespoons neutral oil. Blend until very smooth, scraping down the sides as needed. Add up to 1 more tablespoon of oil if necessary to achieve a pourable paste consistency. Set aside 1/4 cup of this paste for the mousse.
Step 4: Prepare the Gelatin: Sprinkle 1 teaspoon gelatin powder over 2 tablespoons cold water in a small, heatproof bowl. Let it bloom for 5 minutes until softened.
Step 5: Make the Custard Base: In a medium saucepan, whisk together 1/2 cup heavy cream, granulated sugar, and cornstarch until smooth. In a separate bowl, whisk 3 egg yolks. Gradually whisk about 1/2 cup of the hot cream mixture into the egg yolks to temper them, then pour the tempered egg mixture back into the saucepan with the remaining cream. Cook over medium-low heat, whisking constantly, until the mixture thickens to a pudding-like consistency (about 5-7 minutes). Do not boil.
Step 6: Incorporate Gelatin and Black Sesame: Remove the custard from heat. Add the bloomed gelatin and whisk until completely dissolved. Let the custard cool for 10-15 minutes, stirring occasionally, until it's warm but not hot. Stir in the 1/4 cup black sesame paste and vanilla extract until well combined and smooth.
Step 7: Fold in Whipped Cream: In a separate large bowl, whip the remaining 1 cup heavy cream until medium peaks form. Gently fold about one-third of the whipped cream into the cooled black sesame custard to lighten it. Then, carefully fold in the remaining whipped cream in two additions until just combined, being careful not to deflate the mousse.
Step 8: Assemble and Chill: Pour the black sesame mousse into the cooled tart crust. Smooth the top with a spatula. Chill in the refrigerator for at least 4 hours, or preferably overnight, until completely set.
Step 9: Garnish and Serve: Carefully remove the tart from the pan. Garnish with additional toasted black sesame seeds, fresh berries, or edible gold leaf, if desired. Slice and serve cold.