Serves: Makes 1 (9- or 9½-inch) tart shell
- 1 stick (4 ounces) unsalted butter, softened (65° to 68°F)
- ⅓ cup (2¼ ounces) sugar
- 1 large egg yolk
- 1 cup (5 ounces) unbleached all-purpose flour
- 2½ tablespoons unsweetened cocoa powder, preferably Dutch-process
- Place the butter and sugar in the bowl of the stand mixer and beat on medium speed for 3 minutes. The mixture should be smooth and creamy. Scrape down the bowl with the spatula. If there are any lumps of hard butter still visible, beat for another minute to completely blend them into the mixture. Add the egg yolk and beat well. Scrape down the bowl.
- In a the medium bowl, whisk together the flour and cocoa powder. Add to the butter mixture all at once. Turn the mixer to the lowest speed and blend just until the dough comes together (it should still have medium and small clumps) and looks moist, with a dark, uniform color. Scrape down the bowl. If there are any patches of flour or unincorporated butter, finish mixing the dough by hand with the spatula. If you beat too long or too vigorously on the mixer, you will have a smooth, batterlike dough, which will be harder to press into the pan because it is sticky. If this happens, chill the dough for 15 minutes before continuing.
- Scrape the dough clumps into the tart pan. Use the heel of your hand to press the dough into an even layer across the bottom and up the side of the pan. Press from the center of the pan outward, building up some extra dough where the bottom meets the side. Using your thumbs, press this excess up the sides of the pan to form the walls, making sure the dough is level with the pan at the rim. Save any small bits of extra dough in case you need to patch a crack during the baking process. Make sure there are no cracks or thin spots-if there are, just press the dough to bring it together. Chill in the refrigerator for 1 hour or in the freezer for 30 minutes.
Storing: The dough keeps for 3 days in the refrigerator and up to 6 weeks frozen.