Preheat oven to 375 degrees F. For filling, in a large bowl stir together sweet potatoes, granulated sugar, the 1/4 cup maple syrup, the ginger, cinnamon, nutmeg, allspice, and salt. Add eggs; beat lightly with a fork just until combined. Gradually stir in buttermilk until thoroughly combined.
Place the Baked Pastry Shell on a foil-lined baking sheet on the oven rack. Carefully pour filling into pastry shell. Bake for 30 minutes.
Meanwhile, in a small saucepan melt the butter over medium heat. Gradually stir in brown sugar, the 2 tablespoons maple syrup, and the milk. Cook and stir until mixture comes to boiling. With pie on the oven rack, sprinkle partially baked pie with pecans and marshmallows. Carefully pour hot brown sugar mixture over the top. Bake for 15 to 20 minutes more or until center appears set when shaken. Cool on a wire rack for at least 1 hour. Cover and chill within 2 hours.
Tip *To prepare mashed sweet potatoes, in a covered medium saucepan cook about 18 ounces peeled, cubed sweet potatoes in enough boiling salted water to cover for 25 to 30 minutes or until tender. Drain potatoes. Mash potatoes with a potato masher or beat with an electric mixer on low speed until mashed.
Tip **To make 1 cup sour milk, place 1 tablespoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1 cup total liquid; stir. Let stand for 5 minutes before using.
Make Ahead Tip:
Prepare as directed. Cover and chill for up to 48 hours.