Melt butter in a saucepan over medium heat. Cook and stir onion in melted butter until soft, about 5 minutes. Add curry powder, salt, and cinnamon to the onion; cook and stir 1 to 2 minutes more.
Transfer onion mixture to a blender with pumpkin puree and cilantro; blend until smooth. Add yogurt; blend again until smooth and pour into saucepan over low heat. Stir chicken broth and milk through the pumpkin mixture; stirring occasionally, cook until hot, about 10 minutes.
