1 cup flour
1/4 cup cocoa powder
1/4 cup confectioners' sugar
Pinch of salt
1 stick very cold butter, cut into small pieces, plus butter for greasing
1 large egg yolk
1 tablespoon ice water, plus more if necessary
Chocolate Pastry Cream
2/3 cup sugar
1/4 cup cornstarch
Pinch of salt
2 eggs or 4 yolks
2 cups cream, half-and-half, or whole milk
2 ounces dark chocolate, chopped
2 tablespoons unsalted butter, softened
2 teaspoons vanilla extract
Chocolate Ganache Glaze
1/2 cup cream, plus more as needed
4 ounces dark chocolate, chopped
Make the crust:
1. Use a food processor to pulse together the flour, cocoa powder, sugar, and salt to combine. Add the butter and pulse until it has the texture of cornmeal. If you’re making the dough by hand, combine the dry ingredients in a large bowl; then, use your fingertips, 2 knives or forks, or a pastry cutter to mash the butter into the our mixture.
2. Add the egg yolk and pulse to combine (or use a fork to incorporate it), then pulse in the ice water, adding more ice water as necessary, a tablespoon at a time, until the dough starts to form a ball. Turn the dough out onto a lightly floured counter and knead it until it’s just combined and smooth.
3. Gently press the dough into a generously buttered tart pan, being sure to tuck it into the corners. Trim the dough even with the top of the pan. Freeze the crust in the pan for 20 minutes before baking.
4. Heat the oven to 425°F. Be sure the crust is pressed firmly into the pan, pricked all over with a fork, and well chilled before baking; the fork pricks and hardened butter in the dough will help the crust keep its shape. Butter one side of a piece of foil large enough to cover the crust; press the foil onto the crust, butter side down. Scatter your weights in an even layer over the foil and bake for 12 minutes; remove the weights and foil. Reduce the oven temperature to 350°F and continue baking the crust is fully cooked, another 15 minutes or so. Cool completely on a rack before filling.
Make the pastry cream:
1. In a small saucepan, combine sugar with the cornstarch and salt. Mix together the eggs or yolks and the cream. Whisk the cream-egg mixture into the sugar- cornstarch mixture over medium heat. At first, whisk occasionally to minimize the lumps. Then whisk almost constantly until the mixture just begins to boil and thickens, about 10 minutes.
2. Adjust the heat so the mixture bubbles gently, add the chocolate, and continue to cook until it coats the back of a spoon; when you draw your finger through this coating, the resulting line will hold its shape. Stir in the butter and vanilla, strain through a fine-mesh sieve, and set aside. Refrigerate, topped directly with plastic wrap to prevent a skin from forming, until chilled. This sauce can be prepared 1 to 2 days before you plan to use it.
Make the chocolate glaze:
1. Put the cream in a pot and heat it until it’s steaming. Put the chocolate in a heatproof bowl, pour on the hot cream, and whisk until the chocolate is melted and incorporated into the cream.
2. Thin the still-warm ganache with additional cream, about 2 tablespoons at a time, until it’s easily pourable and doesn’t harden too much when cooled to room temperature. To test the consistency, spread a small spoonful on a plate; it should thicken but remain very soft. Use right away as a sauce or coating; as it cools down, it will start to set and get stiffer and harder to spread.
1. Fill the tart crust with the pastry cream to 1/4 inch from the top of the edge; smooth the surface as much as you can. Put it in the fridge or freezer until the surface of the cream chills and stiffens a bit. Use an oiled spatula to spread the glaze over the surface of the whole tart. Chill until set. Serve chilled or at room temperature.