Prepare the dry ingredients for the crust. In a food processor, pulse together flour, cocoa powder, spices, and salt until well combined.
Add the cold butter to the dry mixture. Pulse until the mixture resembles coarse sand with some pea-sized pieces of butter remaining.
Combine the wet ingredients separately. In a small bowl, whisk together brown sugar, molasses, egg yolk, and vanilla until smooth.
Form the dough by gradually adding the wet ingredients to the food processor while pulsing, just until the dough starts to come together. Be careful not to overmix.
Shape and chill the dough. Turn the dough onto a lightly floured surface, form into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.
Preheat your oven to 350°F (175°C) and position a rack in the center of the oven.
Roll out the chilled dough between two sheets of parchment paper to about ⅛-inch thickness and large enough to fit your tart pan.
Transfer the dough carefully to your tart pan, pressing it gently into the corners and up the sides. Trim any excess and patch any tears or thin spots.
Blind bake the crust by lining it with parchment paper and filling with pie weights or dried beans. Bake for 15 minutes, then remove weights and parchment and bake for another 8-10 minutes until firm.
Allow the crust to cool completely on a wire rack while you prepare the filling.
Make the chocolate filling by placing the chopped chocolate in a heatproof bowl.
Heat the cream in a saucepan until it just begins to simmer around the edges (do not boil).
Pour the hot cream over the chocolate and let sit undisturbed for 2 minutes to melt the chocolate.
Gently whisk the mixture until smooth and glossy, then add butter, vanilla, and salt, continuing to whisk until fully incorporated.
Pour the filling into the cooled crust and smooth the top with a spatula.
Refrigerate the tart for at least 2 hours or until the filling is completely set.
Before serving, allow the tart to sit at room temperature for about 15 minutes for the best texture and flavor.
Garnish with finely chopped crystallized ginger if desired, and serve in small slices as it’s quite rich.