For the crust: Use an electric mixer with a paddle attachment to combine flour, cocoa, 1/2 tsp salt, 1/2 tsp cinnamon, and 1/4 tsp cloves. Add butter and mix on low until butter is the size of small peas (about 5 minutes). Add 1 egg and mix until a dough forms. Preheat oven to 350 degrees F. Lightly flour a work surface and roll out dough to just thicker than 1/8 inch. Brush off excess flour and transfer dough to a 10 inch tart pan with a removable bottom. Press dough into the bottom and sides of pan and trim excess flush to the edge. Lightly prick the bottom of the dough all over with a fork. Chill until firm (about 30 minutes). Bake shell until firm (about 15 minutes) and immediately sprinkle 4 oz of the finely chopped chocolate over the bottom of the shell and smooth with a spatula.
For the filling: Preheat the oven to 350 degrees F. Mix together pumpkin, brown sugar, crème fraîche, 3 eggs, 1 tsp cinnamon, ginger, nutmeg, 1/4 tsp salt, and 1/8 tsp cloves until smooth. Pour mixture through a fine sieve set over a clean bowl and discard solids. Pour the filling into the prepared crust. Bake until the filling is set, about 40 minutes. Transfer to a wire rack and let cool at least 30 minutes. Using a double boiler, simmer water and melt remaining 2 oz of chocolate, sitrring occasionally. Remove from heat. Dip a spoon in the melted chocolate, then drizzle chocolate over tart, forming decorative stripes. Refrigerate until the chocolate is well set, for at least an hour.