Whisk chili oil, soy sauce, black vinegar, sesame oil, warm water, and sugar in a small bowl. Stir in red pepper flakes if using.
Mix pork, garlic, ginger, scallions, soy sauce, Shaoxing wine, sugar, salt, white pepper, and egg white. Beat with a spoon until tacky.
Set a small bowl of water nearby and line a sheet pan with parchment for the folded wontons.
Spoon about 1 teaspoon filling onto a wrapper. Moisten edges, fold into a triangle, press out air, and seal tightly.
Bring a large pot of water to a rolling boil over high heat.
Slip in wontons in batches. Simmer 3–4 minutes until they float and the filling is cooked through.
Drain well, then toss hot wontons with a few spoonfuls of sauce. Top with sesame seeds and scallions. Serve immediately.