Heat the oven to 275°F (135°C).

In a small bowl, combine the salt, pepper, garlic powder, smoked paprika, and brown sugar. Stir well, then set aside 2 tablespoons of the mixture for the glaze.

Pat the ribs dry with paper towels. Rub all sides lightly with soy sauce to help the seasoning adhere. Sprinkle the ribs generously with the remaining seasoning mix, pressing it into the meat to create an even coating.

Place a wire rack inside a large roasting pan or rimmed baking sheet. Pour the water into the bottom of the pan. Arrange the ribs on the rack, meaty side up, spacing them evenly. Cover tightly with foil and roast for 2½ hours, uncovering briefly every hour to spritz or brush lightly with a vinegar-based chili sauce or a mix of water and barbecue sauce, if desired.

Meanwhile, prepare the glaze: In a small saucepan, melt the butter over medium heat. Add the reserved seasoning mixture and stir until fragrant, about 30 seconds. Stir in the Korean barbecue sauce and chili vinegar sauce. Simmer gently, stirring often, for 5 minutes, then remove from the heat and set aside.

After 2½ hours, remove the foil and increase the oven temperature to 300°F (150°C). Brush the ribs generously with the glaze, making sure to coat all sides. Return the ribs to the oven, uncovered, and roast for 45 minutes to 1 hour, until the glaze is glossy and slightly sticky. For a more caramelized finish, broil for 2 to 3 minutes at the end, watching closely to prevent burning.

Let the ribs rest for 10 minutes. Slice between the bones and transfer to a serving platter. Spoon over any remaining glaze and garnish with green onions and sesame seeds.