01 - Preheat oven to 350°F. Have ready a 9-inch tart pan with removable bottom.
02 - Whisk together flour, cocoa powder, powdered sugar, and salt in a bowl. Cut in cold butter until mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons ice water; mix until dough comes together. Add more water if needed, 1 teaspoon at a time.
03 - Press dough evenly into the tart pan. Prick bottom with a fork. Freeze for 10 minutes.
04 - Line crust with parchment and fill with pie weights. Bake for 15 minutes. Remove weights and parchment; bake 5 more minutes. Cool completely.
05 - Heat heavy cream in a small saucepan until just simmering. Pour over chopped chocolate in a heatproof bowl. Let sit 2 minutes, then whisk until smooth. Add butter and vanilla; stir until glossy.
06 - Pour ganache into cooled tart shell. Smooth top with a spatula. Chill for at least 1 hour until ganache is set.
07 - Arrange raspberries on top. Dust with powdered sugar if desired. Slice and serve at room temperature.