Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers.
Toss in the cinnamon bark and green cardamom. Let cook about 20-30 seconds. You will see little bubbles forming around the bark.
Add the garlic ginger paste and green chilies. Stand back. Cook until the garlic ginger paste stops spluttering.
Turn the heat to medium low. Add the spice mix. That's the mix you made above and put in the small bowl. Stir continuously until it starts to smell really good. It This takes around 30-40 seconds. Watch it carefully. If you burn the spices at this point you have to start over. This is called blooming the spices. It is one of the big secrets to Indian cooking. It's also why you you need to add all the oil specified in this recipe. Things will go poorly for you if you try shave calories.
Add the diluted tomato paste and stir to combine. Turn the heat up to medium. Cook for 1 minute.
Add half the onion paste and turn the heat up to medium high. Just dump it out of the blender. Stir to combine. Cook for about a minute.
Add the rest of the onion paste and stir again. Cook, stirring occasionally for about 4 minutes. Don't worry if it looks dry. The chicken will release juices as it cooks. Instant delicious.
Turn the heat down to medium low. Add the chicken and stir. Cover and simmer until the chicken is done. It's done when you measure the internal temperature and it says 170F, about 10-15 minutes. I can't tell you how long this takes. I don't know how big your chicken pieces are. I also don't know how hard things are simmering in your pan. An instant read thermometer is a handy thing to have here. It's actually a great thing to have period. Great chefs use them. You should use them too.
If the curry is a bit thick add a bit of water or chicken stock and stir. You are after a saucy consistency like you get in a restaurant.
Add the diced tomatoes and cilantro. Let the chicken masala simmer another minute or so. Basically you are just warming the tomatoes through. Taste for salt and adjust as needed.
Garnish with a bit more cilantro if desired. This dish goes great with a simple tarka dal, chapattis and rice.