Arrange the 10 chicken drumsticks in a single layer in a 13x9-inch baking dish or a resealable plastic bag.
For short marination (2 to 7 hours):In a small bowl, whisk together the 2 tablespoons light brown sugar, 1 cup good-quality soy sauce, and the full 1 cup unseasoned rice vinegar until the sugar dissolves.For long marination (8+ hours or overnight):In a small bowl, whisk together the 2 tablespoons light brown sugar, 1 cup good-quality soy sauce, and ½ cup of the rice vinegar until the sugar dissolves. Pour the mixture over the chicken, ensuring all pieces are coated evenly. Nestle the 2 tablespoons black peppercorns, 10 cloves garlic, 3 Thai red chilies, 2- inch piece of ginger, and 5 bay leaves around the chicken. Cover tightly with plastic wrap or seal the bag and refrigerate for at least 2 hours, or overnight for deeper flavor.
Heat the 2 tablespoons neutral cooking oil in a large Dutch oven over medium heat (Note 6). Remove the chicken from the marinade, letting excess liquid drip back into the dish, and set the marinade aside. Before browning, brush off any peppercorns stuck to the chicken skin to prevent burning (Note 7).
Working in batches to avoid overcrowding the pan, sear the chicken until browned on all sides, about 2-3 minutes per side. Allow the pan to return to temperature between batches for even browning. Avoid using high heat, as it can cause the sugars in the marinade to burn. Transfer browned chicken to a plate as you work.
Once all the chicken is browned, return it to the Dutch oven. Pour the reserved marinade over the chicken. If you marinated the chicken for longer than 8 hours or overnight, add the remaining ½ cup rice vinegar at this point. Bring the mixture to a gentle boil, then reduce the heat to low and cover. Simmer for 30 minutes, turning the chicken halfway through cooking to ensure even flavor absorption.
After 30 minutes, remove the lid and continue cooking uncovered for another 25-30 minutes, turning the drumsticks occasionally, until the chicken is tender and the sauce has thickened and reduced to a glossy consistency. Maintain low heat while simmering to prevent the marinade's sugars from burning. The sauce should coat the back of a spoon (Note 8). Use an instant-read thermometer to ensure the chicken has reached an internal temperature of 165 F (74 C).
Strain the sauce to remove the peppercorns, ginger, bay leaves, and chilies. Transfer the sauce to a serving bowl or pitcher. Serve the chicken with the strained sauce on the side. Garnish with sliced green onions or cilantro, if desired.
Optional Garnish: thinly sliced green onions or cilantro