In a large bowl, whisk together soy sauce, balsamic vinegar, honey, garlic, and spring onions. Set aside 60ml marinade.
Add chicken to a large resealable plastic bag and pour in remaining marinade. Let marinate in the fridge at least 2 hours or up to overnight.
When ready to BBQ, heat BBQ to high. Oil grates and BBQ chicken, basting with reserved marinade and turning every 3 to 4 minutes, until charred and cooked through, 24 to 30 minutes total.