To bake sweet potato: Wash and dry the sweet potato then pierce the skin all over with a fork. Bake in a pre-heated over at 400ºF for 40-50 minutes, until a small knife can be easily inserted into the flesh. Let it cool completely then scoop out the flesh. Set aside.
To make the sweet potato burger buns: Heat the whole milk in the microwave for about 35-45 seconds until it’s warm to the touch but not hot (110-115ºF). Add the active dry yeast and let it dissolve and foam up for 10 minute
In a stand-mixer with a dough-hook, add the bread flour, baked sweet potato, light brown sugar, salt, grated nutmeg and 1 large egg yolk (reserve the egg white for egg-wash later). Add whole milk and yeast, then mix on medium speed for 5 min until the dough is elastic and smooth. If the dough is too dry that it won’t come together after a couple minutes, add 1 tsp of the reserved egg white to moisten.
Then knead the unsalted butter into the dough on medium speed, 1 tablespoon at a time (only add the next addition when the previous one is completely incorporated into the dough). Increase the speed to medium-high and knead the dough for another 6-8 minutes until shiny, elastic and very smooth. The dough should be slightly sticky, but pulls away cleanly from the bowl while the machine’s running.
Place the dough in a large, lightly oiled bowl then cover with plastic wrap. Let proof at a warm/room-temperature spot until fully doubled in size, approximately 2 hours. If you are making the buns the day before using, proof refrigerated for 18~24 hours.
Scrape the dough onto a lightly floured surface and punch out the air. Divide it into 7 equal portions (or 6 if you want it a little bigger). Stretch the surface of the dough and tuck it underneath itself to form a smooth top-surface while shaping the dough into a ball. Place the doughs on a parchment-lined baking-sheet then loosely cover with plastic wrap. Let proof again at room temperature for 45-50 min (or 2 hours if the dough was refrigerated overnight), until the doughs have almost doubled again.
Meanwhile, preheat the oven on 375ºF.
Beat the reserved egg-white with 1 tsp of water to make egg-wash. For an extra shiny finish, brush the doughs with egg-wash, then let dry for 5 min and brush again for another coating. Sprinkle the top with flaky sea salt and fresh thyme leaves, then bake in the oven until golden browned, about 20 minute.