Special equipment: a butane kitchen torch
Preheat the oven to 425 degrees F.
Using a fork, pierce the sweet potato all over, then bake for 1 hour. Cool the potato slightly, then carefully cut in half and scoop out the flesh with a spoon. In a large mixing bowl, combine the potatoes, the 4 tablespoons melted butter, the flour, maple syrup, milk, cinnamon and egg yolks. Stir until smooth.
In a separate, very clean mixing bowl, whip the egg whites with a whisk until stiff peaks form. Using a rubber spatula, gently fold the egg whites into the potato mixture.
Melt the remaining 1 tablespoon butter in a large skillet set over medium-high heat. Use a 1/4 cup measure to scoop out the potato mixture. Form the pancakes with your hands and carefully place in the hot pan. Cook for 1 to 2 minutes per side, then remove from the pan and top each pancake with 2 marshmallow halves. Use a kitchen torch to lightly toast the top of the marshmallows, then serve.