01 - Whisk together flour, cocoa powder, powdered sugar, and salt in a bowl. Add cold butter and rub in with fingertips or a pastry cutter until mixture resembles coarse crumbs. Stir in egg yolk and 2 tablespoons cold water, mixing until dough just comes together, adding more water if needed. Press dough into a disk, wrap in plastic, and chill for 30 minutes.
02 - Preheat oven to 350°F. Roll out dough on a lightly floured surface to fit a 9-inch tart pan. Press dough into the pan, trim excess, and prick bottom with a fork. Line with parchment and fill with pie weights or beans. Bake for 15 minutes, remove weights and parchment, and bake 5 more minutes. Let cool completely.
03 - Place chopped chocolate in a heatproof bowl. Heat cream in a saucepan until just simmering, then pour over chocolate. Let sit 2 minutes. Add butter, then gently stir until smooth and glossy.
04 - Pour ganache into cooled tart shell and smooth the top. Chill for 1 hour or until ganache is set.
05 - Arrange raspberries over the ganache. Optionally, warm raspberry jam and brush gently over berries for a glossy finish.