Preheat oven to 350 degrees. In a medium bowl, whisk together pumpkin, brown sugar, creme fraiche, eggs, cinnamon, ginger, nutmeg, salt, and cloves until smooth. Pass mixture through a fine sieve into a clean bowl (discard solids). Pour filling into prepared crust, just to top edge.
Bake until filling is set, about 40 minutes. Transfer to a wire rack, and let cool at least 30 minutes.
Place chocolate in a heatproof bowl, set over a pan of barely simmering water; heat until melted, stirring occasionally. Transfer chocolate to a parchment paper cone or resealable bag with a tiny hole cut in one corner. Pipe about 15 evenly spaced lines radiating out from center of tart. Pipe curved lines around perimeter of tart, connecting each spoke. Continue piping curved lines, spacing them closer together as you near the center. Refrigerate tart until set, at least 1 hour (or up to 1 day).
Melted semisweet chocolate piped over the top adds a fanciful finish and serves as an excellent guide for slicing.