Preheat oven to 350°F.
To make chocolate crust:In the bowl of an electric mixer fitted with the paddle attachment, combine flour, sugar, cocoa, salt, cinnamon and cloves. Add butter, mix on low speed until butter is the size of small peas, about 30-40 seconds. Add egg and beat until ingredients come together to form a dough.
On a lightly floured work surface, roll out dough to just thicker than 1/8 inch. Brush off excess flour; transfer dough to a 10-inch tart pan with a removable bottom. Press dough into bottom and up sides of pan; trim excess flush with edge. Lightly prick bottom of dough allover with a fork. Chill until firm, about 30 minutes.
Bake shell until firm, about 15 minutes. Immediately sprinkle chocolate over bottom of shell; smooth with a spatula. Leave the oven on.
To make filling:In a medium bowl, whisk together pumpkin puree, brown sugar, crème fraîche, eggs, cinnamon, ginger, nutmeg, salt, and cloves until smooth. Pour filling into prepared and chilled chocolate crust. Bake until filling is set, about 40 minutes. Transfer to a wire rack, and let cool.
To decorate:Set a heatproof bowl, or the top of a double boiler, over a pan of barely simmering water. Melt chocolate in bowl, stir occasionally; remove from heat. Dip a spoon in melted chocolate, then drizzle chocolate over tart, forming decorative stripes. Refrigerate until well set, at least 1 hour and up to 1 day.
Special equipment: 10" tart pan.
Makes 1 ten-inch tart (12 servings)