01 - In a bowl, whisk together flour, cocoa powder, and salt. Add cold butter and rub in with fingertips or a pastry cutter until the mixture resembles coarse crumbs.
02 - Stir in powdered sugar. Add egg yolk and 2 tablespoons cold water; mix until dough just comes together. Add more water if needed.
03 - Shape dough into a disk, wrap in plastic wrap, and chill for at least 30 minutes.
04 - Preheat oven to 350°F. Roll out dough on a lightly floured surface to fit a 9-inch tart pan. Press dough into pan, trim excess, and prick base with a fork.
05 - Line pastry with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, then remove weights and paper and bake an additional 5 minutes. Let cool completely.
06 - Heat heavy cream in a saucepan until just simmering. Pour over chopped chocolate in a bowl and let sit for 2 minutes. Stir until smooth, then add butter and vanilla extract, stirring until glossy.
07 - Pour ganache into cooled tart shell. Chill until set, about 1 hour.
08 - Arrange fresh raspberries evenly on top. For a glossy finish, gently heat raspberry jam and brush over the berries.
09 - Serve chilled or at room temperature.