Heat the oil in a medium frying pan until it just starts to shimmer.
Add the green cardamom, cassia bark and curry leaves. Cook about 30 seconds. You want to see little bubbles forming around your whole spices. It you don't get them right from the start heat your oil a little more next time. What you don't want to do is panic and crank the heat. You will just wind up burning things in the next step.
Stir in the garlic ginger paste and green chilies. Cook until the garlic ginger paste stops sputtering.
Turn your heat to medium low and add your spice mix. This is why you added a full 3 tablespoons of oil at the beginning. You want your spices to fry in the oil. Too little oil and the spices will stick or burn and you will be starting over.
Gently fry the spices in the oil, stirring constantly, for about 30 seconds. This is where magic happens. Volatile compounds in the spices will be released into the oil. And that is big flavour.
Turn the heat up to medium. Add the Indian hotel curry gravy. Bring to a simmer. Really stir it to get the oil to combine and cook for about a minute. You don't need it to fry hard like a regular restaurant curry. That's the beauty of hotel style. You already have that done. No need to make a mess of your stove.
Add the chicken in an even layer and cover the pan. After about 5 minutes flip the chicken pieces. Tongs are good for this. Can't beat a good set of restaurant tongs in the kitche. Cook until the chicken is just done.
The chicken should take around 8-12 minutes to cook through. It really depends on how large the chicken thigh pieces are. Best bet is to use an instant read thermometer and go for 170F.
Once the chicken is done, stir in the coconut milk and tamarind paste. Simmer for one minute.
The texture should be about right at this point. If it's too thick add a bit of water or chicken stock Not a lot. Probably a couple tablespoons max.
If it is too thin (chicken throws a fair bit of liquid as it cooks) just let the curry simmer uncovered for a minute or two.
Taste and decide if you fall in the slightly sweet camp. If you do add the sugar and stir. It will open up the flavours a bit. I like it.
Serve with rice or Indian flatbread. Or both!