Melt the butter in a heavy 3-quart saucepan over medium-high heat. Add the onion, celery, carrot, and garlic puree; cook for 3 minutes, until vegetables are soft. Mix in the potatoes, cinnamon, nutmeg, allspice, and bay leaf, and stir continuously for 2 to 3 minutes, until potatoes begin to soften and spices are aromatic.
Add the chicken stock, bring to a boil, lower the heat to medium-low, and simmer for 30 minutes. Remove the bay leaf. Puree the soup using a hand-held immersion blender, or blend in several batches in a blender. (If using a standard blender, place a folded dish towel over the top to prevent hot liquid from splashing out.) Pulse on and off until the soup is smoothly pureed. Return the soup to the saucepan and whisk in the cream, brown sugar, molasses, salt, and pepper. Serve hot.
Preheat oven to 300 degrees. Place the garlic in a small, ovenproof container and pour in the oil. Use additional oil if needed to completely immerse all the garlic cloves. Cover the container with aluminum foil and roast 1 hour, until garlic is soft and light golden brown.
Strain the garlic and place in a blender along with 2 tablespoons of the oil. Puree to a smooth consistency, adding a small amount of oil if necessary. Pour into a container and cover the top of the puree with a thin layer of the oil. Cover and store in the refrigerator for up to 1 month.
Reserve the remaining garlic-infused oil in another container and refrigerate.
Recipe courtesy of The Heaven on Seven Cookbook by Jimmy Bannos and John DeMers (10 Speed Press, 2001)