Preheat your smoker to 350°F.
Peel your sweet potatoes, then rough-chop into 1” cubes.
Bring a large pot of water to boil and add your cubed sweet potatoes to the water and let them boil for 20 minutes until fork tender.
Strain the sweet potatoes into a colander and return them to the same pot that you boiled them in.
Add the butter, brown sugar, vanilla extract, cinnamon, kosher salt, and black pepper to the pot.
Using a potato masher, mash the sweet potatoes until they reach a smooth consistency and the ingredients are fully-combined.
Transfer the mashed sweet potatoes to a casserole dish or aluminum tray and smooth out the top with the back of a spoon.
Sprinkle the chopped walnuts over the top, then add an even layer of mini marshmallows that fully covers the top of the casserole.
Place your casserole directly on the grates
Let it smoke for 25 to 30 minutes until the tops of the marshmallows start to turn brown.
Remove from the smoker and serve immediately.