How to Make It
Preheat oven to 350°F. Pierce yams several times with a fork, and wrap each in aluminum foil. Bake in preheated oven until tender, 1 hour to 1 hour and 30 minutes. Remove from oven; cool completely, about 2 hours.
Preheat oven to 350°F. Peel cooled yams, and discard skins. Process yams in a food processor until mixture is smooth and forms a ball, about 1 minute. Place 11/2 cups yam puree in a large bowl; reserve remaining puree for another use. Whisk in 1 cup brown sugar, milk, 1/2 cup butter, eggs, vanilla, ube flavoring, 1/2 teaspoon cinnamon, five-spice, salt, and nutmeg until smooth and combined. Pour mixture into frozen pie shell. Bake in preheated oven until filling is set, about 50 minutes.
Meanwhile, stir together oats, pecans, coconut, remaining 2 tablespoons brown sugar, remaining 2 tablespoons butter, and remaining 1/4 teaspoon cinnamon in a bowl. Set aside.
Remove pie from oven; sprinkle center of pie with oat mixture. Return to oven; bake at 350°F until topping is lightly browned, about 10 minutes. Sprinkle marshmallows over topping, and bake until lightly browned, about 5 minutes. Transfer pie to a wire rack to cool completely, about 4 hours.