01 - Preheat oven to 350°F. Grease a 9-inch tart pan with removable bottom.
02 - In a bowl, whisk together flour, cocoa powder, powdered sugar, and salt until combined.
03 - Using a pastry cutter or your fingers, cut cold butter into the dry mixture until it resembles coarse crumbs.
04 - Add egg yolk and cold water. Mix until dough just comes together. Press into the bottom and up the sides of the tart pan.
05 - Prick crust with a fork. Freeze for 15 minutes. Bake for 18-20 minutes until set. Cool completely.
06 - Place chopped chocolate in a heatproof bowl. Heat cream in a small saucepan until just simmering, then pour over chocolate. Let sit 2 minutes, then stir until smooth.
07 - Stir in butter and vanilla extract until glossy and incorporated.
08 - Pour ganache into cooled crust. Smooth the top. Refrigerate for at least 2 hours until set.
09 - Arrange fresh raspberries on top of set ganache. Dust with powdered sugar if desired before serving.