Step 1: Make the tart crust. In a food processor, pulse flour, cold butter, and sugar until mixture resembles coarse breadcrumbs.
Step 2: Add egg yolk and cold water, pulse until dough just comes together. Form into a disc, wrap in plastic, and refrigerate for 30 minutes.
Step 3: Preheat oven to 180°C (350°F). Roll out dough on a floured surface to fit a 23cm (9-inch) tart pan with removable bottom.
Step 4: Press dough into pan, trim edges, and prick base with a fork. Line with parchment paper and fill with baking weights or dried beans.
Step 5: Blind bake for 15 minutes, remove weights and parchment, then bake for another 10 minutes until golden. Let cool completely.
Step 6: Make the ganache. Place chopped chocolate in a heatproof bowl. Heat cream in a saucepan until just simmering.
Step 7: Pour hot cream over chocolate, let sit for 2 minutes, then gently stir until smooth and glossy.
Step 8: Stir in softened butter, vanilla extract, and sea salt until fully incorporated and ganache is silky.
Step 9: Pour ganache into cooled tart shell, smooth the top with a spatula, and refrigerate for at least 4 hours until set.
Step 10: Before serving, garnish with fresh raspberries and dust with cocoa powder. Serve at room temperature.