For this recipe, we prefer the steady, even heat of a cast-iron skillet. A heavy stainless-steel skillet may be used, but you may have to increase the heat slightly.
1. For the Dipping Sauce: Whisk all ingredients together in small bowl; set aside.
2. For the Pancakes: Using a wooden spoon, mix 1 1/2 cups flour and boiling water in bowl to form rough dough. When cool enough to handle, transfer dough to lightly floured counter and knead until tacky (but not sticky) ball forms, about 4 minutes (dough will not be perfectly smooth). Cover loosely with plastic wrap and let rest for 30 minutes.
3. While dough is resting, stir together 1 tablespoon vegetable oil, sesame oil, and remaining 1 tablespoon flour. Set aside.
4. Place 10-inch cast-iron skillet over low heat to preheat. Divide dough in half. Cover 1 half of dough with plastic wrap and set aside. Roll remaining dough into 12-inch round on lightly floured counter. Drizzle with 1 tablespoon oil-flour mixture and use a pastry brush to spread evenly over entire surface. Sprinkle with 1/2 teaspoon salt and half of scallions. Roll dough into cylinder. Coil cylinder into spiral, tuck end underneath, and flatten spiral with your palm. Cover with plastic and repeat with remaining dough, oil-flour mixture, salt, and scallions.
5. Roll first spiral into 9-inch round. Cut 1/2-inch slit in center of pancake. Cover with plastic. Roll and cut slit in second pancake. Place 2 tablespoons vegetable oil in skillet and increase heat to medium-low. Place 1 pancake in skillet (oil should sizzle). Cover and cook, shaking skillet occasionally, until pancake is slightly puffy and golden brown on underside, 1 to 1 1/2 minutes. (If underside is not browned after 1 minute, turn heat up slightly. If it is browning too quickly, turn heat down slightly.) Drizzle 1 tablespoon vegetable oil over pancake. Use pastry brush to distribute over entire surface. Carefully flip pancake. Cover and cook, shaking skillet occasionally, until second side is golden brown, 1 to 1 1/2 minutes. Uncover skillet and continue to cook until bottom is deep golden brown and crispy, 30 to 60 seconds longer. Flip and cook until deep golden brown and crispy, 30 to 60 seconds. Transfer to wire rack. Repeat with remaining 3 tablespoons vegetable oil and remaining pancake. Cut each pancake into 8 wedges and serve, passing dipping sauce separately.
To Make Ahead: Stack uncooked pancakes between layers of parchment paper, wrap tightly in plastic wrap, and refrigerate for up to 24 hours or freeze for up to 1 month. If frozen, thaw pancakes in single layer for 15 minutes before cooking.