1. Preheat oven to 425°F. Roll out pastry into an 11-inch round. Transfer to a 9-inch pie plate. Trim and crimp edges. Line with parchment and fill with pie weights. Bake until firm, about 10 minutes. Reduce oven temperature to 375°F. Remove parchment and weights. Bake until lightly browned around edges, about 7 minutes.
2. Place sweet potatoes in a pot; add salted water to cover by 1 inch. Bring to a boil. Reduce heat slightly; cook potatoes until soft, 15 to 20 minutes. Drain well. Transfer to a large bowl; add butter, sugar, molasses, spices, vanilla and 1/4 tsp. salt. Beat until smooth. Beat in eggs.
3. Reduce oven temperature to 350°F. Spread filling in crust. Bake until firm in center, about 45 minutes. Cool slightly. (Can be made up to 1 day ahead. Cover and refrigerate.)
4. Just before serving, set an oven rack 6 inches from heat source; preheat broiler to high. Spread marshmallows over pie. Watching carefully, broil until marshmallows are golden brown, 1 to 1 1/2 minutes. Serve.