Cut the chicken into bite-sized pieces. Combine the vinegar, tamarind juice, 1/4 cup curry powder, salt and pepper in a bowl. Add chicken and toss to coat.
Heat the coconut oil in a wok or frying pan over medium heat. Cook the sliced onion, green chillies, cardamom pods, cloves, curry leaves, ginger and cinnamon stick until the onion has softened and turned translucent, about 5 minutes. Reduce heat to medium-low and continue cooking and stirring until the onion is very tender and dark brown, 15 to 20 minutes more. Stir in the garlic and cook for an additional minute.
Add the chicken mixture, water and tomato paste. Stir and simmer until the chicken is cooked through, about 10 minutes. Add the roasted curry powder and stir until evenly dispersed.
Gradually stir in the coconut milk and simmer for 2-3 more minutes. (Do not overheat or the coconut milk may curdle.)
Roasted curry powder and tamarind juice are available from specialty purveyors or online sources. If curry leaves are unavailable, you may substitute 2 bay leaves.
To roast your own curry powder, heat 3 tablespoons of curry powder in an ungreased frypan over medium heat, stirring frequently, until fragrant and slightly browned. Immediately transfer the spice to a bowl to cool.