Peel the sweet potato and cut into cubes. Place in a saucepan and add enough water to cover the potato by 2 inches. Bring to a boil and cook until soft, about 10 minutes. Drain and mash with a fork or potato masher. Let cool.

Combine the yeast, 2 teaspoons brown sugar, and warmed milk in a small bowl and whisk to dissolve the sugar. Let sit for 6-8 minutes or until the mixture is foamy and has increased in volume.

Combine the flour, xanthan gum, salt, and baking powder in the bowl of an electric mixer fitted with the whisk attachment. Mix on medium-low to combine. Add the yeast mixture, 2 eggs, melted butter substitute (or butter), 2 tablespoons brown sugar, and mashed sweet potato. Mix on medium-low until combined. Scrape down the sides of the bowl, turn the mixer on high, and mix for 3 minutes. You should have a very thick, smooth batter.

Brush 2 standard muffin pans with melted butter substitute (or butter) or spray with gluten-free nonstick cooking spray. Preheat the oven to 375ºF.

Spoon the batter into the prepared muffin pans, filling the cups about ¾ full. Cover with clean kitchen towels and place in a warm, draft-free place to rise. Let rise for 45 minutes or until the dough has almost doubled in size.

Beat the remaining egg with 1 teaspoon of water very well with a fork. Gently brush the tops of the rolls with the beaten egg. Bake for 17-18 minutes or until golden brown. Let cool in the pans for 15 minutes.

The rolls can be made ahead: Bake them, let them cool in the pans, wrap with plastic wrap, and refrigerate for up to 2 days. Warm for a few minutes in a 350ºF oven before serving.