Step 1: Pat chicken pieces dry with paper towels. Cut into 1-inch bite-sized pieces if not already.
Step 2: In a medium bowl, whisk together the gochujang, soy sauce, rice vinegar, sesame oil, brown sugar, minced garlic, grated ginger, and gochugaru (if using) until well combined.
Step 3: Add the chicken pieces to the marinade, tossing thoroughly to ensure every piece is coated. Cover the bowl and refrigerate for at least 30 minutes, or preferably 2-4 hours, for best flavor. You can even marinate overnight.
Step 4: Heat 1 tablespoon of neutral oil in a large non-stick skillet or wok over medium-high heat. Once the oil is shimmering, add the marinated chicken in a single layer, being careful not to overcrowd the pan. Cook in batches if necessary.
Step 5: Cook the chicken for 5-7 minutes, stirring occasionally, until it is cooked through, caramelized, and the sauce has thickened into a glossy coating. The sugar in the marinade helps create a beautiful sticky glaze.
Step 6: Remove the skillet from heat. Garnish the spicy gochujang chicken with sliced green onions and toasted sesame seeds. Serve immediately.