Step 1: Melt the Butter - Set a large pot over medium heat and add the butter. Let it melt completely, but keep an eye on it so it does not start browning. You want a smooth, mellow base for the soup. Once the butter has melted and is beginning to gently bubble around the edges, you are ready for the garlic and brown sugar.
Step 2: Cook the Garlic and Brown Sugar - Add the minced garlic and packed brown sugar to the melted butter. Whisk everything together for about one minute. The brown sugar will dissolve into the butter and become glossy while the garlic starts to smell warm and savory. This little minute at the beginning matters because it gives the garlic a chance to mellow and helps the brown sugar blend smoothly into the soup instead of sitting in little clumps.
Step 3: Simmer the Broth - Pour in the chicken broth and water, then add the salt and black pepper. Whisk everything together and turn the heat to high. Once the mixture reaches a boil, give it another good stir, reduce the heat to low, and let it gently simmer for about 15 minutes. You do not need to stand over the pot the whole time. I usually use these few minutes to clear the counter, slice bread, or remind somebody for the third time that dinner really is almost ready.
Step 4: Whisk in the Pumpkin and Milk - Carefully add the pure pumpkin, evaporated milk, and cinnamon to the pot. Whisk slowly at first so the pumpkin does not splash, then keep whisking until everything looks smooth and evenly combined. The color will turn into that deep, warm orange that makes this soup feel like fall before you have even tasted it. Take a moment to scrape around the bottom and edges of the pot with the whisk so no thicker pockets of pumpkin are hiding there.
Step 5: Finish the Soup - Continue cooking the soup over low heat for another 5 minutes, whisking occasionally. Keep the heat gentle once the milk has been added. You want the soup hot and silky, not furiously boiling. Taste it before serving and adjust the salt or pepper if needed. Ladle the soup into bowls and, if you like, finish each serving with a small swirl of heavy cream and a handful of croutons. That contrast between creamy soup and crisp croutons is especially good.