For the crust:
- 1 cup all-purpose flour, plus more for dusting
- 1/4 cup plus 1 tablespoons granulated sugar
- 1/4 cup unsweetened cocoa powder
- 1/2 teaspoon salt
- 1/2 teaspoon ground cinnamon
- 1/4 teaspoon ground cloves
- 1/2 cup (1 stick, 8 tablespoons) cold unsalted butter, cut into small pieces
- 1 large egg
- 4 ounces semisweet chocolate, finely chopped
For the filling:
- 1 can (15 ounce) unsweetened pumpkin puree
- 3/4 cup packed light brown sugar
- 1 cup sour cream
- 3 large eggs
- 1 teaspoon ground cinnamon
- 1 teaspoon ground ginger
- 1/4 teaspoon freshly grated nutmeg
- 1/4 teaspoon salt
- 1/8 teaspoon ground cloves
For the topping:
- 2 ounces semisweet chocolate, finely chopped
Make the crust:
Whisk together flour, sugar, cocoa, salt, cinnamon, and cloves in a bowl. Add butter. With an electric mixer on low speed beat until butter is the size of small peas, about 5 minutes. Add egg, mix until ingredients form a dough. Wrap dough in plastic and refrigerate for at least 1 hour or up to 2 days.
Preheat oven to 350 degrees. On a lightly floured surface, roll out dough to just more than 1/8-inch. Press dough into bottom and up sides of a 10-inch tart pan with a removable bottom. Trim excess dough flush with rim. Pierce bottom of shell all over with a fork. Refrigerate or freeze until firm, about 30 minutes or up to 1 day.
Place tart pan on a rimmed baking sheet and bake until dry, about 15 minutes. Immediately sprinkle the 4 ounces chocolate evenly over crust; let it begin to melt, then smooth with an offset spatula.
Make the filling:
In a medium bowl, whisk together pumpkin, brown sugar, sour cream, eggs, cinnamon, ginger, nutmeg, salt, and cloves until smooth. Pass mixture through a fine sieve into a clean bowl, discard the solids. Pour filling into prepared crust, just to top edge.
Bake at 350 degrees until filling is set, about 40 minutes. Transfer to a wire rack and let cool at least 30 minutes.
Make the topping:
Place the 2 ounces chocolate in a microwave-safe bowl and microwave in 15 second bursts, stirring between bursts, until melted and smooth. Transfer chocolate to a resealable bag with a tiny hole cut in one corner. Pipe about 15 evenly spaced lines radiating out from the center of the tart. Pipe curved lines around the perimeter of the tart, connecting each spoke. Continue piping curved lines, spacing them closer together as you near the center. Refrigerate until set, 1 hour or up to 2 days.