Add the butter to a large pot and melt it over medium heat. Whisk in the minced garlic and brown sugar and keep stirring for about a minute, until the brown sugar has liquified.
Add the chicken broth, water, salt, and pepper. Turn the heat to high and bring it to a boil. Stir, then reduce the heat to low and let it cook for 15 minutes.
Carefully add the pumpkin, evaporated milk, and cinnamon. Whisk well to combine.
Cook for 5 more minutes, whisking occasionally.
Garnish bowls with a swirl of heavy cream and croutons if you'd like.
Notes
Storage: Store leftover soup in an airtight container in the refrigerator for up to 4 days or freeze for up to 3 months.