In a large bowl, mix together the ground pork, chopped cabbage, mushrooms, green onions, garlic, and ginger.
Add the soy sauce, sesame oil, oyster sauce, salt, and pepper. Mix well until combined.
Make the Charcoal Dough:
If using store-bought wrappers, skip this step. If making your own, add the activated charcoal powder to your dough mixture and knead well until the color is evenly distributed. Roll out thinly and cut into circles.
Assemble the Gyoza:
Place a spoonful of filling in the center of each gyoza wrapper.
Moisten the edges with a little water, fold the wrapper in half, and pinch to seal. Pleat the edges if desired for a traditional look.
Cook the Gyoza:
Heat vegetable oil in a large skillet over medium-high heat.
Arrange the gyoza in the skillet, flat side down. Cook until the bottoms are golden brown, about 2-3 minutes.
Add a splash of water (around 1/4 cup) to the skillet, then cover with a lid. Steam for 5-6 minutes, or until the water has evaporated and the gyoza are cooked through.
Prepare the Dipping Sauce:
In a small bowl, mix together soy sauce, rice vinegar, sesame oil, and sugar until the sugar dissolves. Add chili or garlic if desired for extra flavor.