Day 1: Evening

1. In a medium bowl, add 25 grams sourdough. Pour in the water and whisk until the sourdough is completely incorporated. If using einkorn flour, start with 100 grams of water and add more as necessary.
2. Add in the flour and sugar. Mix using a Danish dough whisk or wooden spoon until the dough starts to form.
3. Add the oil, and knead to incorporate, until smooth. The dough will be slightly sticky. Cover with a damp towel or plastic wrap, and let rise on counter 30 minutes to one hour.
4. Perform a stretch and fold and return dough to the bowl. Cover tightly with plastic wrap and place in the refrigerator overnight, up to 12 hours.

Day 2: Morning or Afternoon

1. The next day, take the dough out of the refrigerator and allow it to warm up to room temperature for 45 minutes to 1 hour.
2. Sprinkle ½ teaspoon baking powder over the dough. Pour 1 teaspoon of white rice vinegar over the baking powder. Work into the dough using your fingers until it is thoroughly incorporated into the dough. Let the dough proof for 45 minutes to 1 hour.
3. Cut parchment paper into 10 four inch squares. You could also use lettuce leaves. (The purpose is to keep the buns from sticking to the steamer). The first time I made the buns, I used parchment squares, for this batch, I used a parchment circle that fit nicely inside the bamboo steamer. I also greased the parchment so the buns didn’t stick to it.
4. Deflate the dough and divide it into 10 equal pieces (about 43 grams each). Give each piece a quick knead.
5. On a floured surface roll the dough out into a 3 inch by 6 inch rectangle with rounded edges. Fold the dough in half lengthwise, grease lightly with oil, (to help the buns open up easily without tearing) and place on a parchment squares or circle. You may not be able to steam them all at once, depending on the size of your steamer.
6. Cover lightly with oiled plastic wrap or a damp towel, and repeat with the rest of the dough pieces. Let proof for 30 to 45 minutes, until slightly puffy.
7. Bring a pot or wok of water to a steady boil (just slightly more than simmering) and fit your pan or wok with a steamer, bamboo basket, or steaming rack just above the water. Place the baos in the steamer, cover, and steam for 12 minutes. Cool slightly, fill with a filling of your choice, and eat.