Preheat the oven to 425 degrees F. Poke a few holes in the sweet potatoes with a fork. Place them on a baking sheet and bake until potatoes are tender, about 45 to 60 minutes.
Let the potatoes cool slightly, then slice them down the center lengthwise. Gently scoop out the sweet potato flesh, leaving the skin intact so you can stuff it.
I like to spray the skin with olive oil, sprinkle it with salt and pop them back into the oven for 10 to 15 minutes so they really crisp up. It makes the skin so delicious!
Place the sweet potato in the bowl of an electric mixer. (Note: you can mash it with a fork, but I find that whipping it in a mixer makes it super silky and fluffy!) Add the salt, butter and maple syrup. Mix on medium speed until combined and smooth.
Scoop the mixture back into the sweet potato skins. Top with the marshmallows. Bake for 15 to 20 minutes, or until potatoes are warmed through and the marshmallows are melty. Remove the potatoes from the oven and top with cinnamon and nutmeg. Sprinkle with flaked salt. Serve!
Note: you can bake the sweet potatoes ahead of time and store in the fridge until you're ready to use!