Preparation:

Step 1: Mix the dough

In a large mixing bowl, whisk together the sourdough starter, milk, and egg until no traces of egg or large lumps of starter remain. Add sugar, salt, and flour into the milk mixture. Stir the mixture using your hands or a wooden spoon until it forms a shaggy mass and no dry flour remains. Incorporate the butter into your dough one tablespoon at a time. Your dough will start out oily and sticky right after each addition, but will progressively feel smoother and less tacky as you knead it. Don’t add more butter until the previous addition is fully absorbed into your dough. Continue kneading for 1 or 2 more minutes, then shape your dough into a tight ball and place it into a clean bowl that has been lightly coated in oil. Cover the bowl with a dough cover and let it rest for 30 minutes.

Step 2: strengthen dough and bulk fermentation

Turn your dough out onto a lightly floured surface. Knead your dough until it feels strong and extensible, about 5 minutes. Gather your dough into a tight ball and put it back into your bowl, cover and let it rise untouched until it has doubled in volume and looks airy and feels like it’s full of bubbles. Once your dough has doubled, knock the air out with your fist, round it into a tight ball and re-cover your bowl. Place your dough into the refrigerator to rest overnight.

Step 3: Make the filling

While your dough bulk ferments, make the filling. Heat two tablespoons of oil in a large heavy bottomed pot over medium high heat. Add your meat to the pot and brown it on all sides. Once your meat has fully browned, about 1 minute on each side, remove it from the pot and set aside. Fry the onions in the same pot until soft and caramelized, about 5 to 10 minutes, taking care not to allow them to burn. While your onion cooks, mix together the water, soy sauce, hoisin sauce, and brown sugar in a large measuring cup, set aside. Add the garlic into the pot with the onion and cook for another minute or until the garlic is aromatic and lightly browned.

Put the pork back in the pot, pour the sauce over and add the bay leaves, salt and pepper. Bring the sauce to a rolling boil for about a minute then turn the heat down to a gentle simmer. Cover the pot and braise the pork until it’s tender, about 1 and a half to two hours. You’ll know the pork is done once it is tender enough to break apart with a fork. Fish bay leaves out of your stew and discard.

Dissolve the cornstarch in 30 grams of water, bring the pork back to a boil and add the cornstarch solution. Simmer uncovered for another 30 minutes or until the sauce has thickened and reduced by half: your sauce should have the consistency of a rich, thick gravy. Allow your filling to cool to room temperature before storing it in an airtight container in the refrigerator overnight.

*This recipe makes more filling than you’ll need, enjoy any leftovers with rice!

Step 4: Shape and proof

The next day, take your dough and bun filling out of the refrigerator. Make sure to bring your filling up to room temperature (about 78 F), as filling your dough with cold filling will drastically slow down proofing. Heating it in the microwave for 30 seconds or so works great, just make sure it doesn’t get hotter than 115 F, or it will kill your yeast. Using two forks, shred the filling into chunks or varying sizes. Line your Challenger Bread Pan with a piece of parchment paper.

Using your bench knife, divide your dough into 10 equal portions. Round each portion into a tight ball, cover the dough portions with plastic wrap and let them rest for 15 minutes. This will make shaping easier. Working with one portion at a time, flatten the dough into a 5 inch circle using a rolling pin. Place 25 grams of filling in the center, crimp the dough closed making sure it’s tightly sealed so your filling does not leak out while your buns are baking. Lay the bun seam side down on your prepared Challenger Bread Pan. Repeat with the rest of the dough portions and arrange your unbaked buns into 3 rows. Cover the dough with a lightly-oiled piece of plastic wrap and allow them to rise at room temperature until they look puffy and well risen, about 4 hours.

Step 5: bake

Preheat your oven to 375 F. Brush the buns with egg wash and top with sesame seeds, if using. Bake the buns until they are golden brown and baked through, about 25 to 30 minutes. Remove the buns from the oven and allow them to cool for about 15 minutes before serving.