Whisk gochujang, soy sauce, brown sugar, rice vinegar, sesame oil, garlic, and ginger in a large bowl until smooth. Reserve 3 tablespoons of marinade in a small bowl for glazing later.

Add chicken thigh pieces to the remaining marinade and toss to coat evenly. Cover and refrigerate for at least 30 minutes or up to 4 hours. The longer it sits, the deeper the flavor.

Thread 4-5 chicken pieces onto each skewer, leaving small gaps between pieces for even cooking. Let excess marinade drip off. If using bamboo skewers, soak them in water for 30 minutes first to prevent burning.

Heat grill or grill pan to medium-high (about 400 degrees F). Brush grates with vegetable oil. Grill skewers for 12-15 minutes total, turning every 3-4 minutes, until chicken is charred in spots and registers 165 degrees F internally.

In the last 2 minutes of grilling, brush reserved marinade over the chicken. The glaze should bubble and caramelize slightly. Remove from heat and let rest 2 minutes.

Transfer to a platter, sprinkle with toasted sesame seeds and sliced scallions. Serve with steamed rice and quick pickled cucumbers if desired.