Whisk 2 cups flour and ½ tsp salt in a large bowl. Mix in sesame oil and boiling water with a spoon or spatula until a shaggy dough forms.
Then add the cool water and mix until it is incorporated.
Turn out dough onto a lightly floured surface and knead, until dough is smooth, about 5 minutes (adding a pinch of flour at a time if the dough is sticky)
Cover and let rest at room temperature 1 hour.
Divide dough into 8 pieces. Working with one at a time, roll out on a lightly floured work surface as thin as possible (about 1/8 inch thick)
Brush each disc with melted butter, a pinch of salt and about 2 tbsp scallions, leaving some space on the end without scallions so you can seal it up.
Roll the dough into a thin cylinder, then, starting at 1 end, roll the cylinder to create a coil (like a cinnamon roll). Cover and repeat with remaining dough. Let rest at room temperature 15 minutes.
Working with 1 coil at a time, roll out on a lightly floured surface to a 5" round (keep other coils covered). Repeat with remaining dough.
Heat 1 Tbsp. vegetable oil in a medium skillet over medium-low. Cook pancakes, turning frequently to prevent scallions from burning, until golden brown and crisp on both sides and cooked through, 8–10 minutes. Transfer pancakes to a wire rack and let rest about 5 minutes before cutting into wedges.
Optional: Make the Sauce. Whisk soy sauce, vinegar, sugar, and red pepper flakes in a small bowl until sugar is dissolved. Serve alongside pancakes for dipping.