How to Make It
In a heatproof bowl, soak the tamarind pulp in the boiling water, covered, until softened, 30 minutes. Strain through a coarse sieve; discard the seeds.
Meanwhile, in a small skillet, combine the peppercorns, dried red chiles, coriander seeds, cumin seeds and cinnamon and toast over moderate heat until fragrant, 1 minute. Transfer to a spice grinder to cool. Grind to a fine powder.
Heat the oil in a large enameled cast-iron casserole. Season the chicken with salt and pepper. Add half of the chicken to the casserole and cook over moderately high heat until browned, about 4 minutes per side. Transfer to a plate and repeat with the remaining chicken.
Pour off all but 2 tablespoons of the fat from the casserole. Add the onion and cook over moderately low heat, stirring occasionally, until softened, about 6 minutes. Add the garlic, ginger, turmeric, ground spices and green chiles. Cook, stirring, until fragrant, 3 minutes.
Stir the water and the strained tamarind pulp into the casserole. Return the chicken to the casserole and bring to a boil, then simmer over low heat for 20 minutes. Transfer the chicken breasts to a plate. Add the coconut milk and simmer for 20 minutes longer. Transfer the remaining chicken to the plate and simmer the sauce until very flavorful, about 8 minutes. Season with salt and pepper. Return the chicken to the sauce. Simmer the chicken until heated through before serving.