1. Preheat oven to 400° F. Bake sweet potatoes for 1 hour or until tender, covering the oven dish with aluminum foil.
2. Reduce oven temperature to 350° F.
3. Let sweet potatoes cool, then peel their skin off and place them into a mixing bowl.
4. Add Grappa, brown sugar, butter, vanilla, salt, cinnamon, and the egg.
5. Beat with an electric mixer until smooth.
6. Transfer the sweet potato mixture to a greased baking dish.
7. Sprinkle the pecan nuts on top and Bake for 30 minutes.
8. Remove from oven and sprinkle marshmallows on top. Return to oven for 10 minutes.