Boil sweet potato in 2 cups of water in a saucepan until fork tender and almost falling apart, 15-20 minutes. Reserve cooking water.
1 sweet potato
Remove sweet potatoes with a slotted spoon, reserving the water, and set aside to cool slightly.
Measure out ½ cup of flour into stand mixer bowl. Measure ½ cup of the boiling water and pour over the ½ cup flour in the mixing bowl. Mix until combined and pasty. Set aside to cool. Discard remaining water, if any.
3 ½ cups all-purpose flour
To the mixing bowl with the flour paste, add the slightly cooled sweet potato, 3 remaining cups of flour, sour cream, sugar, 2 eggs, yeast, and salt. Start mixer on low with dough hook attachment. Mix until the dough comes together to form a smooth dough and pulls away from the sides of the bowl, adding extra flour a tablespoon at a time if the dough seems too sticky.
1 sweet potato, 3 ½ cups all-purpose flour, ½ cup sour cream, 2 tablespoons granulated sugar, 2 large eggs, 2 ¼ teaspoons instant dry yeast , 1 teaspoon salt
With the mixer still running, add 6 tablespoons of butter, one tablespoon at a time, waiting for each piece to incorporate before adding the next piece. This part takes patience. Stick with it, it’ll be worthwhile! After all the butter has been incorporated, continue to mix for a few more minutes until you have a smooth and supple dough.
9 tablespoons unsalted butter
Add 1 tablespoon vegetable oil to a large clean bowl. Coat sides of the bowl with oil. Remove the dough from the mixing bowl and form the dough into a smooth ball with oiled hands to prevent sticking. Place the dough into the greased bowl. Cover with plastic wrap and let rise until doubled in size about an hour*. Then place in the fridge for 8-12 hours.
1 tablespoon vegetable oil
Grease a 9x13-inch pan with 1 tablespoon of butter and set aside.
9 tablespoons unsalted butter
Lightly flour a clean work surface and turn the dough out onto it. Gently press the dough into a square and divide the dough into 15 evenly-sized pieces. Shape the dough one piece at a time into smooth balls by bringing the edges into the center, pressing to seal. Place the dough ball seam side down on a clean work surface and rotate your hand around it to smooth out the ball. Place in the greased pan. Dough balls may be touching.
Cover and let proof until doubled in size about 1 hour*. Meanwhile, preheat the oven to 350ºF.
Lightly whisk the remaining egg in a small bowl. Gently brush the buns with the egg wash. Sprinkle with flaky salt if desired.
1 large egg, flaky salt
Bake in the preheated oven until the tops are golden brown, about 20-25 minutes. Remove from the oven and immediately glaze with the remaining 2 tablespoons of butter by rubbing the butter over the hot buns. Serve rolls warm.
9 tablespoons unsalted butter