01 - Combine flour, cocoa powder, powdered sugar, and salt in a food processor. Add cold butter and pulse until coarse crumbs form. Incorporate egg yolk and 2 tablespoons cold water, blending until dough holds together. Add an additional tablespoon of water if necessary. Shape dough into a disk, wrap, and refrigerate for 30 minutes.
02 - Preheat oven to 350°F (175°C). Roll dough to fit a 9-inch tart pan. Press into the pan, trim edges, and prick the base with a fork. Line with parchment and fill with pie weights or dried beans. Bake 15 minutes, remove weights and parchment, then bake 5 minutes more. Cool completely.
03 - Heat heavy cream in a saucepan until just simmering. Pour over chopped chocolate in a heatproof bowl, let sit 2 minutes. Stir until smooth, then add butter, vanilla, and salt. Stir until glossy.
04 - Pour ganache into cooled crust and smooth the surface. Allow to set at room temperature for 10 minutes, then refrigerate for 1 hour until firm.
05 - Top with fresh raspberries and optionally dust with powdered sugar. Slice and serve chilled or at room temperature.