Preheat oven to 325 degrees F (165 degrees C).
Mix the graham cracker crumbs together with 1/3 cup white sugar, butter, and 1/8 teaspoon salt in a mixing bowl. Spread evenly over the bottom and sides of a 9 inch pie pan.
Bake the graham cracker crust in preheated oven until lightly browned, about 5 minutes. Remove from oven, and cool on a rack.
Preheat oven to 400 degrees F (200 degrees C).
To make the sweet potato filling, place the mashed sweet potatoes in a mixing bowl. Whisk in the brown sugar, whipping cream, eggs, vanilla, cinnamon, mace, and salt until smooth and evenly blended. Pour the sweet potato mixture into the baked graham cracker crust.
Bake the pie in preheated oven until the center is set, about 45 minutes. Remove from oven and cool on a rack.
To make the marshmallow meringue, place the marshmallows and milk in a pan; cook over low heat, folding mixture gently until marshmallows are half melted. Remove pan from the heat. Continue folding until marshmallows melt completely, and the mixture becomes smooth and fluffy.
Place the egg whites in a separate mixing bowl, and beat until soft peaks form. Gradually add 1/4 cup white sugar, and continue beating until stiff peaks form. Beat in vanilla and salt. Gently fold the egg whites into the marshmallow mixture, and spread over the top of the pie.
Return the pie to the oven and cook until the meringue is toasted, 5 to 10 minutes.
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