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