Prepare the black sesame filling:

1. Toast the black sesame seeds in a heavy-bottomed pan over medium heat, stirring continuously, until fragrant and beginning to pop, about 5 minutes.** Transfer the toasted seeds to a plate and allow to cool slightly, then place in a food processor or coffee grinder along with the sugar and grind to a fine meal. Reserve ¼ cup (35g) of the ground black sesame for the dough.
2. For the regular filling: transfer the remaining ground black sesame to a small bowl and add the sugars, salt, and softened coconut oil. Stir to form a crumbly and sandy black paste. Place in the refrigerator to chill as you prepare the dough. For the molten filling: add the sugars and salt to the remaining ground black sesame in the food processor and pulse an additional minute until fine and pulverized. Transfer to a quart-sized Ziploc bag and add the softened coconut oil and water. Stir or massage (if using a ziploc bag) until fully combined, using the warmth of your hands to melt the coconut oil; the consistency will be wet and thick, like batter. Spread the paste out flat to a ½-inch thickness. Cover or seal the bag and freeze for at least 2-3 hours until solid. Divide into 10 equal balls, about 26 grams each.

Make the dough:

1. In a small bowl or liquid measuring cup, proof the yeast by stirring together the warm water, sugar, and yeast. Let sit for 5 to 10 minutes, until the surface looks foamy. If the yeast does not foam, then start over with newer yeast. If you’re using instant dry yeast, simply stir the ingredients together to combine.
2. In a large bowl, combine the flour, cornstarch, and baking powder. Make a well in the center of the dry mixture. Pour in the wet mixture and stir with a pair of chopsticks until it forms fat flakes. Clean off the chopsticks and add the oil and the reserved ground black sesame to the dough. Knead for 5 minutes, incorporating any remaining flour on the sides of the bowl, until the dough forms a firm, shaggy ball. Continue kneading for another 5 minutes, until the dough is smooth, supple, and elastic. STAND MIXER OPTION: Combine the flour, cornstarch, and baking powder in the bowl of a stand mixer fitted with the dough hook. On low speed, slowly pour in the yeast mixture, then add the oil and reserved ground black sesame. When the dough comes together into a ball, increase the speed to medium and knead for 6 to 8 minutes..
3. Place the dough in a warm place, cover it with a clean tea towel, and let rise until it has doubled in size. This will take 1 to 1½ hours, depending on the room temperature.

Shape the buns:

1. Scrape the risen dough onto a lightly floured surface and knead vigorously for another 5 minutes, to eliminate air bubbles. I like to shape the dough into a log, fold into thirds, and knead, then repeat two or three times until the dough is smooth and elastic. Don’t be afraid of over-kneading; this extra step is important so the buns rise evenly and have a smooth surface.
2. Divide the dough into 10 equal portions, each about 45-50 grams each. Cut out ten squares of parchment paper. On a floured surface, flatten each piece with your palm into a flat, round disk. Using a thin rolling pin, roll the disk into a flat circle about 3½ inches in diameter. Repeat with all the dough portions.
3. To fill the buns, spoon 2 heaping tablespoons of the filling (for regular buns) or place one frozen ball of filling (for molten buns) into the center of one dough portion. Gather or pleat the sides of the dough to enclose the filling and pinch firmly to seal. This is especially important for the molten buns so that the liquid filling doesn’t leak out when steaming. Flip the bun so the seam side is down and shape gently with your palms into a round sphere. Place on a square of parchment paper and set aside. Repeat until you have 10 buns.

Steam the buns:

1. Transfer the buns (on their parchment squares) into two steamer baskets, making sure they’re at least 1 inch apart, as they will expand. Cover and let the buns rest for 5 minutes. Bring water in a pot or wok to a boil over high heat. Place the baskets over the water and steam the buns for 8-10 minutes.
2. Allow the buns to cool slightly before serving. Enjoy with hot tea or a cup of hot soymilk.