First, make pumpkin puree: Take pumpkin, cut in half and scoop out seeds and membrane. On foil-lined baking sheet, place pumpkin halves face down; put in oven and bake at 350°F until tender, about 40 minutes. Let cool about 10 minutes before attempting to scoop out flesh. In heavy saucepan or dutch oven over medium heat, heat oil. Add carrot, celery, onion, garlic and marjoram and sauté until onions begin to wilt, about 5 minutes. Add broth and pumpkin puree. Lower heat and simmer for 15 minutes. Remove from heat and let cool. Using blender, puree cooled soup in batches. Return blended soup to saucepan and add chorizo. Over medium heat, simmer until thoroughly heated. Stir in evaporated fat-free milk and cook 2 minutes. Season with salt and pepper, to taste. Garnish with parsley, if desired.