Make the spice mix.
Dilute the tomato paste with enough water to get to the consistency of passata.
Heat your frying pan (don't use non-stick) briefly over medium heat. Add the oil (you can use some of the oil from frying the shrimp). Use all the oil specified. It's important. You need that oil to bloom the spices. You don't want them to stick and potentially burn because you skimped on a bit of oil.
When the oil starts to shimmer add the cardamom and cinnamon stick. You should see little bubbles form around them. Let that go for around 20 seconds.
Now add the mustard and cumin seed along with the curry leaves. Stir and cook until the seeds start to crackle. This takes another 20 seconds or so.
Add the garlic ginger paste and large pieces of the two green chilies to the pan and cook, stirring constantly, until the paste stops sputtering.
Turn down the heat and add the spice mix. This is the moment of truth in any curry. Stir it constantly for 30 seconds. If it starts to darken lift the pan off the heat. You want the spice mix to cook in the oil but not burn. If it burns you are starting over. No way around it.
Turn the heat up to medium high. Add the diluted tomato paste immediately and stir until bubbles form (the oil will likely separate). This takes around 30 seconds to one minute depending on the heat.
Add 3 oz of curry base. Stir until you see small craters forming, around 30-45 seconds. If it takes much longer that's a sign that you can turn the meat up a bit more. Watch the edges of the pan. The curry can stick here. That's not a problem. Just scrape it back into the pan.
Now add 6 oz of curry base and stir briefly. Let it cook until the craters form again. This takes 1-2 minutes.
Add the rest of the curry base and let cook until the craters form. Turn the heat down to low. Add the coconut milk and stir.
Mix in the remaining diced green chili. Simmer for about 3-4 minutes.
Add the diced tomatoes. Stir to combine. Add the prawns and cook until they are just warmed through.
Serve with rice and/or parathas or chapatis.