Step 1: To a food processor, add the baking powder and 2 cups of the flour. With the machine running, add ⅔ cup of cold water and process until a dough forms, about 40 seconds. Transfer to a plate and set aside. Add the salt and remaining flour to the food processor and, with the machine running, add ⅔ cup of boiling water and process until a shaggy dough forms, about 30 seconds. Return the reserved dough to the food processor and pulse until both doughs come together, about 35 seconds.

Step 2: Lightly flour a work surface and turn the dough out onto it. Knead the dough until smooth and elastic, about 4 minutes. Grease a large bowl with vegetable oil, then transfer the dough to it. Cover and set aside at room temperature until pliable and relaxed, about 2 hours.

Step 3: Lightly flour a work surface and rolling pin. Split the dough in half and roll one half into a 10- by 20-inch rectangle. Brush the rectangle with 1 tablespoon each of the vegetable and sesame oils. Sprinkle with half of the scallions, half of the red chile flakes, and half of the white pepper. Beginning on a long side, tightly roll up the rectangle like a jelly roll, stretching gently outward as you roll. Cut the roll crosswise into 3 even pieces. Lightly stretch one piece and, starting from one edge, coil it horizontally, tucking the outer end beneath it. Gently flatten the coil into a disk with your hand, then flatten with the rolling pin into a 6-inch circle. Repeat with the remaining pieces and the second half of the dough and fillings. Cover the pancakes with a large sheet of plastic wrap and set aside for 10 minutes.

Step 4: Position a rack in the center of the oven and preheat to 200°F. Place a large baking sheet on the rack.

Step 5: To a medium nonstick skillet over medium heat, add 2 teaspoons of vegetable oil. When it’s hot and shimmering, add one pancake and cook, swirling the skillet and flipping the pancake halfway through, until golden and crisp, about 10 minutes total. Using a wide spatula or tongs, transfer the pancake to the baking sheet in the oven. Repeat with the remaining oil and pancakes.

Step 6: Cut the pancakes into wedges and serve hot with soy sauce for dipping.