Preheat your oven to 425°F. Spray a 9-inch round tart pan with a removable bottom with non-stick cooking spray and set it aside.
In a large mixing bowl, whisk together the flour, dark cocoa powder, sugar, and salt. Make a well in the center and pour in the melted butter. Stir until the mixture forms a crumbly dough.
Press the dough into the bottom and up the sides of the prepared tart pan. Place the tart pan on a baking sheet and bake the crust for 10 minutes. The crust may bubble slightly as it bakes. Remove from the oven and let it cool for 10 minutes while you prepare the filling.
In a separate mixing bowl, whisk together the pumpkin, sweetened condensed milk, egg, pumpkin pie spice, and salt until smooth and well combined.
Pour the pumpkin filling into the pre-baked crust.
Bake the tart at 425°F for 10 minutes, then reduce the oven temperature to 350°F and continue baking for an additional 25-30 minutes, or until the filling is set and a toothpick inserted into the center comes out clean.
Allow the tart to cool for at least 1 hour before serving. If desired, serve with a dollop of whipped cream.