In a small bowl, combine the coriander, cumin, black pepper, turmeric, kashmiri chili powder, garam masala and salt. This is your spice mix.
Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers. Add the cinnamon stick. Once small bubbles form and it crackles a bit, add the green chilies and curry leaves. Cook for about 30 seconds.
Add the garlic ginger paste and cook until it stops spluttering.
Turn the heat to medium low. Add the spice mix and stir continuously until it starts to smell really good - around 30-40 seconds. Watch it carefully. 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 1 minute.
Add the onion paste and turn the heat up to medium high. Cook, stirring occasionally for about 5-6 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.
Turn the heat down to medium low. Add the coconut milk and cook for another 3-4 minutes.
Add the pre-cooked chicken. Simmer until the chicken is warmed through - another 2 minutes or so.
Garnish with a bit of cilantro and additional diced green chili pepper if desired.