Dough: Sift the flour with the cocoa and salt. Cream the butter and sugar in the bowl of a mixer fitted with a paddle on low speed until fluffy.
Add the egg, and mix just until incorporated. Add the dry ingredients, and mix until a dough forms. Shape into a ball, cut in half, flatten both halves, wrap each in plastic, and chill for at least 1 hour.
Working quickly, roll out one of the dough balls on a large piece of lightly floured parchment paper to form a 13-inch circle about 1/8 inch thick.
Place the parchment on a baking sheet, cover the dough with plastic, and chill it for at least 1 hour. Keep the remaining dough in the freezer for another tart.
Preheat the oven to 350 degrees.
Carefully fit the dough into a 9½-inch tart shell with a removable bottom, taking care not to stretch or pinch it. Freeze the dough for at least 15 minutes.
Bake the crust until it puffs, about 10 to 15 minutes. Flatten the base by pressing it down with a measuring cup, then continue baking until the crust is crisp, about 10 to 15 minutes more. Set aside to cool.
Chocolate Custard: Preheat the oven to 250 degrees. Place chocolate in a bowl. Bring the cream, milk, and sugar to a boil in a saucepan.
Add the mixture, a third at a time, to the chocolate, stirring each addition until combined. (If it’s added all at once, the chocolate will become grainy.) Beat the egg in a large bowl.
Whisk in a quarter of the chocolate-milk mixture until just combined, and add to the remaining chocolate. (Do not whisk hard.).
Tap the bowl to get rid of any air bubbles. Pour the custard into the baked tart shell, filling it to the top.
Bake for 20 minutes, rotate front-to-back, and bake for another 20 minutes or until the custard is set around the edges but jiggly in the center. Serve with brandy whipped cream.