In a small saucepan, bring the water, sugar, and rum to a simmer and cook until the sugar is dissolved and the mixture is syrupy. Add in the pumpkin and stir to combine. Pour into a large flat dish (like a cake tin) and freeze for one hour. Check the mixture and scrape any formed ice crystals with a fork. Continue to check on the mixture every 45 minutes to an hour until completely frozen and all the crystals are scraped up.
Combine cream, sugar, and molasses in a mason jar and shake until a soft-peak whipped cream forms. Refrigerate until ready to use.
Divide granita into cups or dishes, top with a spoonful of molasses cream, and a sprinkle of cacao nibs.