In a large bowl, combine chicken, mushrooms, cilantro, green onions, garlic, soy sauce, sesame oil, ginger, rice wine vinegar, pepper and red pepper flakes, if using.*
To assemble the dumplings, place wrappers on a work surface. Spoon 1 tablespoon of the chicken mixture into the center of each wrapper. Using your finger, rub the edges of the wrappers with water. Fold the dough over the filling to create a half-moon shape, pinching the edges to seal.*
Heat 2 tablespoons vegetable oil in a large cast iron skillet over medium heat. Working in batches, add potstickers in a single layer and cook until beginning to crisp on the bottom, about 2-3 minutes. Working quickly, add 1/4 cup water; cover and cook until liquid has evaporated and bottoms of dumplings are crisp and golden, about 2-4 minutes.