01 - Combine flour, cocoa powder, powdered sugar, and salt in a mixing bowl. Add cold cubed butter and rub in with fingertips or use a pastry cutter until mixture resembles coarse crumbs.
02 - Add egg yolk and 2-3 tablespoons ice water, mixing gently until dough just comes together. Shape into a disk, wrap in plastic, and refrigerate for 30 minutes.
03 - Roll out dough on lightly floured surface to fit a 9-inch tart pan. Press dough into pan, trimming edges evenly. Prick base all over with a fork.
04 - Freeze crust for 10 minutes while preheating oven to 350°F. Line with parchment paper, fill with pie weights or dried beans, and bake for 15 minutes. Remove weights and continue baking 10 minutes until set. Cool completely.
05 - Heat heavy cream in saucepan until just simmering. Pour over chopped chocolate in a heat-proof bowl. Let stand 2 minutes, then stir until smooth. Add butter and vanilla, mixing until glossy.
06 - Pour ganache into cooled crust, smoothing top with offset spatula. Refrigerate for 1 hour until completely set.
07 - Arrange fresh raspberries atop ganache. For glossy finish, warm raspberry jam and gently brush over berries. Dust with powdered sugar if desired before serving.