Mix the soy sauce, chili flakes, garlic, brown sugar, spring onions, and chopped chilies if you are using them in a large bowl. Stir well until the sugar dissolves.
To pickle the perilla leaves, lay them on a plate or glass container. Pour the pickling sauce over the perilla leaves until they are fully coated. Cover the container, let it sit at room temperature for about four hours, or store it in the fridge for up to three days.
After pickling, the perilla leaves will turn greenish-brown, and the sauce will soak in. Garnish with sesame seeds.