Combine flour and 1/4 tsp salt in a bowl. Add boiling water while stirring, then add cold water and 1 tbsp vegetable oil. Knead until smooth, about 3-5 minutes.
Shape dough into a ball, place in bowl, cover with plastic wrap, and rest at room temperature for at least 30 minutes.
While dough rests, slice scallions thinly. Make dipping sauce by combining soy sauce, rice vinegar, sesame oil, sugar, and optional chili and garlic.
Divide rested dough into 4 equal portions. Roll one portion into a thin rectangle about 20cm x 25cm (8 x 10 inches).
Spread 1.5-2 tsp toasted sesame oil over the dough. Sprinkle with salt and scatter 1/4 of the scallions evenly, leaving a small border.
Roll the dough tightly into a log, stretching gently. Coil the rope into a spiral and flatten into a disc. Roll into a 15-18cm (6-7 inch) round.
Repeat steps 4-6 with remaining dough portions.
Heat skillet over medium heat with 2 tbsp oil. Cook each pancake for 2-3 minutes per side, covered, until deep golden and crisp.
Keep cooked pancakes warm in a 95°C (200°F) oven while frying the remaining ones.
Slice each pancake into 6 wedges and serve immediately with the dipping sauce.