Combine flour, cocoa powder, sugar, and salt in a food processor or large bowl; pulse or whisk to combine. Add cold butter and pulse until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, pulsing until the dough just comes together. Avoid overmixing.

Gather the dough into a disc, wrap tightly in plastic wrap, and chill in the refrigerator for at least 30 minutes.

On a lightly floured surface, roll out the chilled dough to an 11-inch circle. Carefully transfer the dough to a 9-inch tart pan with a removable bottom, pressing it into the sides. Trim any excess dough.

Prick the bottom of the crust with a fork, then line it with parchment paper and fill with pie weights or dried beans. Blind bake at 375°F (190°C) for 15 minutes, then remove weights and parchment and bake for another 5-7 minutes until set. Let cool completely.

In a saucepan, heat heavy cream over medium heat until it just simmers. Remove from heat and stir in chopped dark chocolate until completely melted and smooth.

In a large bowl, whisk together pumpkin puree, brown sugar, eggs, vanilla extract, pumpkin pie spice, cinnamon, and salt.

Gradually whisk the melted chocolate mixture into the pumpkin mixture until well combined and smooth.

Pour the pumpkin-chocolate filling into the cooled, pre-baked tart crust.

Bake at 350°F (175°C) for 45-55 minutes, or until the edges are set and the center has a slight jiggle.

Let the tart cool completely on a wire rack, then chill in the refrigerator for at least 2 hours before serving.