- Using a food processor or standup mixer fitted with the paddle attachment, blend the flour, cocoa powder, brown sugar and salt. Add the butter until the mixture is an even crumbly texture. In a small dish, whisk the egg yolk, milk and vanilla and pour it into the flour mixture, blending until the dough fully comes together. Shape the dough into a disc, wrap and chill it until firm, at least 1 hour.
- On a lightly floured work surface gently knead the chilled chocolate dough to soften slightly. Roll out the dough into a circle about 12-inches across and about ¼ -inch thick. Line an ungreased 9-inch removable-bottom tart pan with the pastry and trim away any excess dough. Chill the tart shell for 30 minutes.
- Preheat the oven to 350°F. Place the chilled tart shell on a baking tray and dock the bottom of the pastry with a fork. Bake the tart shell for about 25 minutes, until the pastry has an even dull appearance. Cool the shell while preparing the filling.