Place sweet potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until very tender, 15 to 20 minutes. Drain well. Mash sweet potatoes with a potato masher. Measure out 1 cup and allow to cool to room temperature. Refrigerate remaining mashed sweet potatoes for another use.

Whisk together 1/2 cup flour, 1/2 cup warm water, and yeast in a large bowl. Cover the bowl and place in a warm place to allow yeast to activate, about 15 minutes.

Spoon 1 cup of cooled mashed sweet potatoes into the yeast mixture and transfer to the bowl of a stand mixer. Add 1 egg, melted butter, honey, and sea salt; mix well. Add 3 cups flour and mix using the dough hook. Mix on medium speed for 2 to 3 minutes. If dough seems too sticky, gradually add in remaining 1/4 cup flour, or as needed, until dough pulls away from the sides of the bowl. Continue kneading until dough is very soft, only slightly sticky, and somewhat elastic, about 2 minutes.

Transfer dough to a floured surface. Shape into smooth ball. Return dough to the bowl and drizzle with oil; rub oil over entire surface to prevent dough from drying out. Cover the bowl and place in a warm spot until dough has doubled in size, about 2 hours.

Meanwhile, line a rimmed baking sheet with a silicone baking mat.

Turn dough out onto a lightly floured surface. Gently press out air pockets. If needed, use only enough flour to keep dough from sticking. Press into a rectangular shape, about 3/4-inch thick. Cut dough into 16 equal-sized pieces. Gently shape each piece into a ball. Slightly flatten each ball and place onto the prepared baking sheet. Let rise until doubled in size, about 45 minutes.

Preheat the oven to 400 degrees F (200 degrees C).

Whisk together remaining egg and 1 teaspoon water in a small bowl until well combined. Brush the tops of buns with egg mixture. Sprinkle dough with sesame seeds.

Bake in the preheated oven until browned, about 15 minutes.