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 tbsp ice water, mixing gently until dough comes together. Shape into a disk, wrap in plastic, and refrigerate for 30 minutes.
03 - Roll out dough on a lightly floured surface to fit a 9-inch tart pan. Press dough into pan, trim edges, and prick base with fork tines.
04 - Freeze crust for 10 minutes while preheating oven to 350°F. Line with parchment paper, fill with pie weights, and bake for 15 minutes. Remove weights and bake 10 additional minutes until set. Cool completely.
05 - Heat heavy cream in a saucepan until just simmering. Pour over chopped chocolate in a heat-proof bowl. Let sit for 2 minutes, then stir until smooth. Add butter and vanilla, mixing until glossy.
06 - Pour ganache into cooled crust, smoothing top with an offset spatula. Refrigerate for 1 hour until completely set.
07 - Arrange fresh raspberries over ganache. For glossy finish, warm raspberry jam and gently brush over berries. Dust with powdered sugar if desired.