For the Pumpkin Puree: Preheat oven to 350. Cut pumpkin in half. With a spoon scoop out all seeds and fibers and use these to make pepitas. Place pumpkin on baking sheet and roast until fork tender, 30-45 minutes. Remove pumpkin flesh from skin. Place in a food processor and process until smooth.
For Pepita Salt Rim: In a food processor pulse until coarse. If mix is moist, add salt.
Margarita: In a sauce pan add pumpkin puree, water, brown sugar, allspice and cinnamon stick. Simmer over low heat for about 20 minutes, whisk to help sugar dissolve. Remove from heat, cool completely.
Using a fine mesh strainer, strain the pumpkin puree (what you are looking for is the pumpkin liquid). Do not toss the thick mix left in the strainer, you might need a tsp or so later.
On a small plate spread pepita salt mix or salt. With lime moisten rim of glasses, dip into salt and place in freezer to chill.
In a pitcher combine tequila, pumpkin liquid and Cointreau. Add lime juice tsp by tsp until you reach desired taste.
If you feel your margarita mix needs an extra punch of pumpkin add a bit of the pumpkin puree left in the strainer. I added an extra two tablespoons to mine, adjust to your taste.