Make the batter: Whisk flour, salt, and water in a bowl until completely smooth. Set aside.
Make the sauce: Mix sweet bean sauce, chili garlic sauce, rice vinegar, and sugar in a small bowl. Taste and adjust.
Heat the pan: Ensure your non-stick skillet (10-12 inches) is completely cold. Do not add any oil and do not turn on the heat yet.
Spread the batter:Pour about 1/2 cup of batter into the center of the cold pan. Lift and tilt the pan in a circular motion to coat the bottom evenly. This is the key trick — spreading on a cold, un-oiled pan gives you time to get an even, thin layer before the batter sets.
Cook the base: Turn the heat to medium-low. Cook until the batter changes from opaque white to translucent, about 1-2 minutes.
Add the egg: Crack an egg onto the crepe. Use a spatula to break the yolk and spread it evenly over the surface.
Add aromatics: While the egg is still wet, sprinkle sesame seeds, scallions, and cilantro over the top.
Flip: Once the egg is mostly set and the edges curl up, carefully flip the crepe with a wide spatula.
Sauce and fill: Brush the surface generously with the sauce. Place potato chips, lettuce, and sausage or ham (if using) in the center.
Fold and serve: Fold the crepe into a packet. Slide onto a plate and eat immediately!