Preheat oven to 400 degrees and line a standard muffin tin with 12 paper liners or spray generously with non-stick spray.

In a medium bowl, whisk together the gluten-free flour, xanthan gum, baking soda, salt, cinnamon, and ground nutmeg. Whisk in the chopped pecans if using then set aside.

In a large bowl, whisk together the brown sugar, eggs, oil, sweet potato purée, milk, vanilla extract, and vinegar. Add the dry ingredients to the wet ingredients and stir until completely combined.

Transfer the batter to the prepared baking pan and distribute them evenly between the 12 muffin cups. Bake the muffins at 400 degrees for 16-18 minutes or until baked through.

Cool the muffins for 5 minutes in the pan and then transfer to a cooling rack. Cool completely before adding the glaze.

To make the glaze, mix the marshmallow fluff and 1/4 teaspoon milk or water together in a small bowl. Add an additional 1/4 teaspoon liquid if the marshmallow isn't thin enough to drizzle from a spoon. Drizzle the marshmallow glaze over the muffins before serving.

Store the muffins at room temperature in an air-tight container for up to 3 days. See notes for freezing instructions.