01 - Combine flour, cocoa powder, powdered sugar, and salt in a food processor. Add cold butter and pulse until coarse crumbs form. Add egg yolk and 2 tablespoons cold water. Pulse until dough starts to come together, adding up to 1 tablespoon more water if needed. Shape dough into a disk, wrap, and chill for 30 minutes.
02 - Preheat oven to 350°F. Roll dough to fit a 9-inch tart pan, press in, and trim edges. Prick 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. Allow crust to cool completely.
03 - Heat heavy cream until just simmering and pour over chopped chocolate in a heatproof bowl. Let sit 2 minutes, then stir until smooth. Incorporate butter, vanilla, and salt; mix until glossy.
04 - Pour ganache into cooled crust and smooth surface. Let set at room temperature for 10 minutes, then refrigerate for 1 hour until firm.
05 - Garnish with fresh raspberries and dust with powdered sugar if desired. Slice and serve chilled or at room temperature.