Cut chicken thighs into bite-sized pieces (about 1.5-inch chunks). Pat dry with paper towels, then season lightly with salt and pepper. In a medium bowl, whisk together soy sauce, sesame oil, minced garlic, minced ginger, and brown sugar until sugar dissolves. Add chicken pieces and toss to coat completely. Let marinate for 10 minutes.

Rinse jasmine rice under cold water in a fine-mesh strainer until water runs mostly clear. In a medium saucepan, bring 2 cups water to a rolling boil. Add rinsed rice, stir once, then immediately reduce heat to lowest setting. Cover with tight-fitting lid and cook undisturbed for 15 minutes. After 15 minutes, turn off heat but keep lid on for 5 more minutes to steam. Fluff gently with a fork.

Heat a large Dutch oven or deep skillet over medium-high heat. Add 1 tablespoon neutral oil and let it heat until shimmering (about 30 seconds). Add marinated chicken in a single layer, leaving excess marinade in bowl. Let cook without moving for 3 to 4 minutes until golden-brown with caramelized edges. Flip pieces and cook another 3 to 4 minutes until chicken reaches 165°F internal temperature.

Pour in chicken broth and rice vinegar, scraping up any browned bits from bottom of pot. Bring to a boil, then reduce heat to maintain gentle simmer. Stir cornstarch mixture, then slowly drizzle into simmering broth while stirring constantly. Broth will thicken within 1 to 2 minutes into a glossy consistency. Simmer 5 more minutes.

Taste broth and adjust seasoning with additional salt, pepper, or soy sauce if needed. Divide rice among four deep bowls. Ladle chicken and broth generously over rice. Top with sliced green onions and serve immediately.