Melt butter in a large pot over medium heat. Add onion, garlic, salt, and pepper; saute until onions are nicely browned, 5 to 7 minutes. Stir in curry powder, cumin, and cinnamon. Increase heat to medium-high, add water and chicken broth, and bring to a boil. Add sweet potatoes and cauliflower, reduce heat, and simmer until vegetables are tender, 25 to 30 minutes.

Puree soup with an immersion blender until nice and smooth, adding more water if too thick for your liking. Cook until heated through, 3 to 5 minutes.