Heat oven to 375 degrees. Line a rimmed 18 by 13-inch baking sheet with a sheet of aluminum foil.
Pierce each sweet potatoes several times with a fork. Place them on prepared baking sheet and bake until soft, about 50 to 70 minutes.
Remove potatoes from oven and let cool slightly. Leave oven on at 375 degrees.
Meanwhile make the topping. In a medium mixing bowl whisk together pecans, flour, brown sugar, 1/4 tsp cinnamon and a pinch of salt. Pour in 3 Tbsp melted butter then stir with a fork until evenly moistened.
Slice each sweet potato in half through the length. Scoop out the flesh of 9 of the halves into a large mixing bowl while leaving about a 1/4-inch rim of sweet potato around each. Scoop out all of the flesh of the last sweet potato (no need to leave a rim) into the bowl (discard the last skin).
Place the 9 hallowed out sweet potato skins on a foil lined baking sheet.
Pass sweet potato flesh through a potato ricer or food mill if you want them completely smooth, otherwise just mash well with a potato masher.
Add the softened butter, heavy cream, maple syrup, 1/4 tsp cinnamon, nutmeg and 1/2 tsp salt. Stir to blend well and then taste and add a little more maple syrup or salt if desired.
Spoon the mashed sweet potato mixture back into the 9 hallowed out sweet potato skins and spread even.
Sprinkle the streusel topping evenly over each sweet potato. Return to oven and bake 10 to 15 minutes until topping is crisp and golden brown.
Remove from the oven then move the oven rack up about 6 inches from the broiler.
Sprinkle the marshmallows evenly over the potatoes (you may have to press slightly so they don't roll off). Heat the broiler then place the baking sheet with the potatoes under broiler and keep there very briefly until marshmallows are golden brown - this may only take about 10 seconds (or up to 1 minute) so don't walk away!
Remove from oven and serve while the twice baked sweet potatoes are warm.
Alternatively you can toast the marshmallows with a kitchen torch, just be very careful as they can ignite quite easily and need to be blown out quickly.