Trim excess fat from the chicken thighs and cut them into bite-sized chunks.

Place the cut chicken in a bowl, add milk, and mix well to coat.

Cover the bowl and refrigerate for 30 minutes to tenderize the chicken.

Finely mince the garlic and ginger and place them in a small bowl.

To the minced garlic and ginger, add soy sauce, rice syrup, and ground black pepper. Mix well.

After 30 minutes, drain the milk from the chicken and pat the pieces dry with a paper towel.

Pour the prepared sauce over the chicken and mix until all pieces are well-coated.

Heat a grill pan over medium-high heat. Add cooking oil, then arrange the chicken on the pan in a single layer.

Turn the heat down to medium, cover the pan, and cook for 5 minutes.

Uncover, add any leftover sauce from the bowl, flip the chicken pieces, and turn the heat up to medium-high.

Continue to cook for another 5 minutes, stirring and flipping the chicken occasionally.

Add butter to the pan. Stir and coat the chicken as the butter melts, cooking for another minute or two until the sauce thickens.

Chop a green onion for garnish.

Transfer the cooked chicken to a serving plate and sprinkle with the chopped green onion.