Whisk together the mashed potatoes, 1 1/2 cups of the broth, ginger, allspice, and nutmeg in a medium saucepan and bring to a simmer over high heat. Reduce the heat to low and cook, stirring occasionally, for 10 minutes. Whisk in the 1/4 cup creme fraiche and more broth, if needed, and cook for 5 minutes.
Whisk together the remaining 1/4 cup of creme fraiche and chipotle puree in a small bowl, and season with salt and pepper. Ladle the soup into bowls and top with a dollop of the creme fraiche, some cilantro leaves and some of the tortilla chips.
Mashed sweet potatoes:
Combine the butter, milk, cream and chipotle puree in a small saucepan and bring to a simmer.
While the potatoes are hot, run them though a ricer into a large bowl. Add the warm milk mixture, syrup, honey, cinnamon and salt and pepper and mix until combined.