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