Arrange pumpkin halves on a baking sheet, cut sides down.
Roast in preheated oven until tender, about 1 hour. Set aside until cool enough to handle, though still warm.
Adjust oven heat to 375 degrees F (190 degrees C).
Scoop flesh from pumpkin halves into a bowl; mash with potato masher. Measure 2 cups mashed pumpkin into a large mixing bowl; add sugar, amber rum, egg yolk, pumpkin pie spice, butter, dark molasses, and vanilla extract. Puree the mixture with an immersion blender or electric hand mixer until smooth and thick. Pour the puree into the pie crust.
Bake in preheated oven until a toothpick inserted into the center comes out clean, 50 to 60 minutes.