Preheat the oven to 400°F and line a standard muffin tin with 12 paper liners or spray generously with gluten-free nonstick cooking spray.

In a medium bowl, whisk together the 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, and vinegar. Add the dry ingredients to the wet ingredients and stir until completely combined.

Transfer the batter evenly between the 12 muffin cups. Bake 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 ¼ teaspoon milk or water together in a small bowl. Add an additional ¼ 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.

Note: To freeze, cool the muffins to room temperature and skip the glaze. Once cool, transfer the muffins to a freezer bag and freeze for up to 3 months. Defrost at room temperature before eating.