01 - Preheat oven to 350°F. Grease a 9-inch tart pan with removable bottom. In a bowl, combine flour, cocoa powder, powdered sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs. Incorporate egg yolk and 2 tablespoons ice water until dough just forms, adding more water if necessary. Press dough evenly into pan, prick base with fork, and freeze 10 minutes.
02 - Bake crust for 15 minutes until set but not browned. Remove and allow to cool completely.
03 - In a saucepan over medium heat, warm heavy cream, milk, and butter until just simmering. Remove from heat and add chopped chocolate; let stand 2 minutes then stir smooth. Stir in sugar, vanilla, salt, and whisk in eggs until mixture is glossy and fully combined.
04 - Pour filling into cooled crust. Bake at 350°F for 15 to 18 minutes until edges are set and center slightly wobbly.
05 - Cool tart on wire rack then refrigerate for at least 1 hour or until firm.
06 - Garnish tart with fresh raspberries and dust with powdered sugar if desired before serving.