In a small bowl, combine the South Indian spice mix, the black pepper, the kashmiri chili powder and the kosher salt.
Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers. Add the green chilies and curry leaves if using. Cook for about 30 seconds.
Add the garlic ginger paste and cook until it stops spluttering.
Turn the heat to medium low. Add the powdered spices (5 tsp south Indian spice mix, 1 tsp kashmiri chili powder, 1/2 tsp of coarse black pepper and the salt) and stir continuously until it starts to smell really good - around 30-40 seconds. Pay attention. If you burn the spices at this point you have to start over.
Add the diluted tomato paste and stir to combine. Turn the heat up to medium. Cook for about 1 minute.
Add the onion paste and turn the heat up to medium high. Cook, stirring occasionally for about 4-5 minutes. Cover it loosely. It will splatter. If it doesn't your heat is too low. The curry will darken a bit as it cooks. Don't worry if it starts to look a little dry. The chicken in the next step will release those tasty, tasty juices and thin everything out so you get a lush sauce at the end.
Turn the heat down to medium low. Add the chicken and stir. Cover and simmer for about 5 minutes. Flip each piece of chicken and continue cooking until the chicken is done. You know it's done when it reaches an internal temperature of around 170F. I can't tell you exactly how long this will take. I don't know how big your chicken pieces are. Shouldn't take more than 10-12 minutes. Don't rush this step. You want a nice gentle braise.
Add the coconut milk and the tamarind paste and cook for about a minute.
Garnish with additional diced green chilies if desired.
Serve with rice and parathas. Parathas are amazing with this dish.