Spiced Sweet Potato Pie with Marshmallow Topping

9-inch unbaked deep-dish pie crust
2 medium sweet potatoes, about 1.5 pounds
6 tablespoons unsalted butter, softened
1 cup packed dark brown sugar
3 tablespoons molasses
1 teaspoon ground cinnamon
3/4 teaspoon ground allspice
1/2 teaspoon ground or freshly grated nutmeg
2 teaspoons vanilla extract
1/2 teaspoon salt
2 large eggs, at room temperature
1 to 2 cups miniature marshmallows
Instructions

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