1. Prick the sweet potato all over with a fork. Place on a microwave-safe plate and microwave at least 4 minutes, turning halfway through, until a knife glides easily through the tender center. Cut in half and let cool before scooping out the flesh and discarding the skin.
2. Press the flesh through a potato ricer for the best texture. You can also puree in a blender or food processor. Or, mash very well until no lumps are remaining. You should have about 230 grams of sweet potato after removing the skin.
3. Combine the buttermilk, butter, eggs, honey, yeast, and sweet potato in the bowl of a stand mixer. Add the salt and 2 cups of the flour and stir with a wooden spoon until the dough forms a rough, shaggy mass.
4. Attach the dough hook to the mixer, turn to medium-low speed, and gradually add in the remaining flour, kneading until a mass of dough begins to forms. Continue kneading on medium-high speed for 4 to 5 minutes until a soft, smooth ball of dough is formed. The dough should feel elastic and slightly tacky to the touch. If the dough is unbearably sticky, add more flour 1 tablespoon at a time.
5. Lightly spray a large clean bowl with cooking spray and place the dough in the bowl. Cover the bowl lightly with plastic wrap. Let rise for about 45 minutes to 1 hour at room temperature or until the dough is big, puffy, and about doubled in size.

Bake the rolls:

1. Spray a 13×9-inch baking pan with cooking spray. Gently deflate the dough. Use a bench scraper, knife, or pizza wheel to divide the dough into 15 equal pieces. Shape each piece into a ball and place in the prepared pan. Beat the egg with 1 tablespoon water. Brush the rolls all over with the egg wash and sprinkle with coarse salt. Lightly cover the dough with plastic wrap and let the rolls rise for 30 minutes, or until about doubled in size.
2. Meanwhile, preheat the oven to 375°F.
3. Bake the rolls for 15 to 20 minutes, or until golden brown. Serve warm. Store leftovers in a plastic bag for up to 3 days.