In a large bowl or container, make the brine for the chicken. Whisk together the coconut milk, 2 tablespoons fish sauce, 1 teaspoon Thai red curry paste, and 1 teaspoon Kosher salt until combined (double these amounts if doubling the recipe). Add chicken pieces to the brine and turn until well coated. Cover and refrigerate for at least two hours or up to 24 hours (the longer the better), flipping in the brine at least once during this time.
When the chicken is finished brining, Preheat oven to 425°F.
Drain or lift chicken from the brine, brushing off as much liquid as possible. Pat all sides dry well with paper towels. Transfer chicken onto a parchment-lined sheet pan, skin sides up, and let come to room temperature.
In a medium bowl, combine 1 tablespoon fish sauce, 9 teaspoons Thai red curry paste, 2 ½ teaspoons Kosher salt, 3 teaspoons agave, 3 tablespoons avocado oil, garlic, and freshly ground black pepper (double these amounts if doubling the recipe). Stir until well combined.
Brush the curry paste mixture over all sides of the chicken, as well as under the skin.
Roast on the middle rack until browned and cooked through, 40-50 minutes. If chicken isn’t browning well or getting crispy, move to the top rack for the last 5-10 minutes. Serve immediately with the Creamy Cilantro Sauce on the side. Garnish chicken with cilantro leaves.