In an 8-quart stockpot, melt the butter and oil together over medium-high heat. Add the shallots and garlic. Season with salt and pepper and cook until soft, about 3 to 4 minutes. Add the sweet potatoes, rosemary and chicken broth. Season with salt and pepper, to taste. Bring the mixture to a boil, reduce the heat and simmer until the sweet potatoes are very tender, about 20 to 25 minutes. Turn off the heat and remove the rosemary stems. Using an immersion blender, blend the mixture until smooth and thick. Whisk in the mascarpone cheese and maple syrup until smooth. Season with salt and pepper, to taste. Keep the soup warm over low heat until ready to serve.
Cook's Note: The soup can also be pureed by ladling, in batches, into a food processor or blender and blended until smooth.