In a food processor, pulse together flour, cocoa powder, powdered sugar, and salt until combined. Add the cold butter cubes and pulse until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, pulsing until the dough just comes together. Do not overmix.

Gather the dough into a disk, wrap it in plastic, and refrigerate for at least 30 minutes.

On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Carefully transfer it to a 9-inch tart pan with a removable bottom, pressing it gently into the bottom and up the sides. Trim any excess dough.

Preheat oven to 375°F (190°C). Line the tart shell with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes, then remove the weights and parchment and bake for another 5 minutes, until the crust is lightly set. Let cool slightly.

Melt the chopped dark chocolate in a heatproof bowl set over a saucepan of simmering water, or in the microwave in short bursts, stirring until smooth. Set aside to cool slightly.

In a large bowl, whisk together pumpkin puree, heavy cream, brown sugar, eggs, vanilla extract, pumpkin pie spice, and salt until smooth.

Slowly whisk the slightly cooled melted chocolate into the pumpkin mixture until fully incorporated and smooth.

Pour the filling into the pre-baked tart crust. Carefully transfer the tart to the oven.

Bake for 30-35 minutes, or until the edges are set but the center still has a slight jiggle. The tart will continue to set as it cools.

Let the tart cool completely on a wire rack, then refrigerate for at least 2 hours before slicing and serving. This allows the tart to firm up properly.