Stir together flour, sugar, yeast, and salt in a large bowl. Slowly add water; using a chopstick, stir water into flour mixture until dough is flaky, about 2 minutes. Knead until a smooth ball forms, about 4 minutes. Cover dough, and let stand at room temperature 1 hour.

In a separate large bowl, stir together ground pork, egg, soy sauce, sesame oil, oyster sauce, dashi powder, ginger, and white pepper until well combined.

Uncover dough (it should be soft and smooth), and roll on a well floured work surface into an 18-inch-long cylinder. Cut dough evenly into 12 (1 ½-inch) pieces.

Using your hands, roll each piece of dough into a ball. With a rolling pin, roll each ball into a 3 ½-inch round, keeping center of each round at least ⅛-inch thicker than outer circumference. (This helps prevent dough from tearing when filling and wrapping.)

Place a pinch of scallions into center of each dough round, and top with a heaping tablespoonful of pork filling. Using your thumb and index finger, pinch and fold dough to close; twist top to seal. Repeat with remaining dough and filling. Any leftover filling can be stored in an airtight container in refrigerator, and reserved for another use.

Heat olive oil in a large deep skillet with a lid over medium-high. Place pork buns in skillet, flat side down, spacing at least 1 inch apart. (Do not overcrowd skillet. If all 12 buns do not fit in skillet, cook in 2 batches.) Cook, undisturbed, until bottoms are slightly brown and release from pan when moved, 1 to 2 minutes.

Carefully add water to skillet, covering buns halfway up sides. Cover pan with lid, and cook over medium-high until water fully evaporates and a thermometer inserted in center of buns registers 160°F, about 15 minutes. Transfer buns to a plate, and serve immediately with chili oil or a soy dipping sauce.