Peel the sweet potatoes and slice very thinly into rounds (or use a mandoline). Brush the cups of a 12-cup muffin tin with some of the melted butter. Stack 5 to 6 slices sweet potato in the bottom of each muffin cup.
Combine the melted butter, brown sugar, vanilla, cinnamon and nutmeg in a small bowl. Spoon a teaspoon of the butter/brown sugar mixture over each of the small stacks of potatoes. Fill the muffin tin with the remaining sweet potatoes. Top each stack with the remaining brown sugar and butter mixture, then bake for 40 minutes.
Remove the muffin tin from the oven and top each muffin cup with a tablespoon of the marshmallow creme and a sprinkle of the pecans. Return to the oven and bake until the marshmallow creme is heated through and starts to melt down the stacks, an additional 5 minutes.
Leave the sweet potato stacks in the muffin tin for 10 minutes while they set and cool slightly. Carefully use a spoon to lift out each stack and transfer to a serving dish. Spoon any additional caramel sauce from the muffin tin over the stacks.