01 - Whisk flour, powdered sugar, and salt in a bowl. Cut in cold butter using a pastry cutter or fingertips until the mixture resembles coarse crumbs. Mix in egg yolk, then add cold water one tablespoon at a time until dough forms.
02 - Press dough evenly into bottom and sides of a 9-inch tart pan with removable bottom. Prick base with fork and refrigerate for 30 minutes.
03 - Preheat oven to 350°F. Line crust with parchment paper and fill with pie weights or dried beans. Bake 15 minutes. Remove weights and paper, then bake another 10 minutes until golden. Cool completely.
04 - Place chopped chocolate in a heatproof bowl. Heat cream in saucepan until simmering; pour over chocolate and let sit 2 minutes. Whisk until smooth, then stir in butter, vanilla, and salt until glossy.
05 - Pour ganache into cooled tart shell, smooth surface, and refrigerate at least 1 hour until firm.
06 - Top with fresh berries, flaky sea salt, or whipped cream as desired before serving.