How to Make It
In a small skillet, toast the coriander, chile powder and turmeric over moderate heat until fragrant, about 1 minute. Stir in the water to form a paste.
In a very large, deep skillet, heat the vegetable oil. Add the onions, garlic, ginger and curry leaves and cook over moderately high heat, stirring occasionally, until the onions are translucent, about 5 minutes. Add in the spice paste, garam masala and black pepper and cook over moderate heat, stirring, until the oil separates from the paste, about 2 minutes.
Lightly season the chicken pieces with salt. Add the chicken, diced potato and squash to the skillet and stir to coat with the seasonings. Stir in the coconut milk and chicken stock and bring to a simmer. Cover and cook over moderately low heat until the chicken is cooked through and the potato and squash are tender, about 25 minutes. Using a slotted spoon, transfer the chicken pieces to a bowl. Season the curry sauce with salt, pour it over the chicken and serve.