Preheat your pellet grill (or oven) to 350 degrees F.
Peel the sweet potatoes and slice very thinly. You can even use a mandolin to slice the sweet potatoes evenly. Brush the cups of a muffin tin with some of the melted butter. Stack 5-6 slices of sweet potato in the bottom of each muffin cup.
In a small bowl, combine the melted butter, brown sugar, cinnamon, nutmeg, and vanilla. Spoon a teaspoon of the butter/brown sugar mixture over each of the small stacks of potatoes. Fill the rest of the muffin tins with the remaining sweet potatoes.
Top each stack with the remaining brown sugar and butter mixture then place in the grill/oven. Bake for 40 minutes.
Remove the muffin tin from the grill or oven and top each with a tablespoon of the marshmallow fluff and a sprinkle of the pecans. Return to the grill and cook for an additional 5 minutes until the marshmallow fluff is heated through and starts to melt down the stacks.
Remove the pan from the grill and leave the sweet potato stacks in the muffin tin for 10 minutes while they cool slightly and set. Carefully use a spoon to lift out each stack and transfer to a serving dish. You can also spoon any additional caramel sauce from the muffin tin over the stacks.