Heat your frying pan (don't use non-stick) briefly over medium heat. Add the oil.
Once the oil starts to shimmer add the panch phoran and mustard seed mix. You should see little bubbles forming around the spices. Cook 15-20 seconds.
Now add the garlic ginger paste. Cook, stirring constantly, until it stops sputtering. This should take 30 seconds or so.
Add the powdered spice mix. This is the critical step. 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. You can go a little longer if you want. Really cook out the spices. Just be careful. Burnt spices are not tasty.
Turn the heat up to medium high. This is important. The heat is what caramelizes (maillard reaction really) the onions in the curry base and gives the curry it's Indian restaurant flavour. As you become more comfortable with this technique try pushing it. Add the diluted tomato paste and stir until bubbles form (the oil will likely separate). This takes around 30 seconds to one minute depending on the heat.
Add around 3 oz of curry base (you don't need to be super precise here). Stir until bubbles form (little craters really), around 30 seconds. Think lively boil. Watch the edges of the pan. The curry can stick here. Sticking is OK. Just scrape it back into the base. Burning is bad.
Now add 6 oz of curry base and stir briefly. Let it cook until the bubbles form again. This takes 1-2 minutes.
Add the rest of the curry base and let cook until the bubbles form. Turn the heat down to low and add the pre-cooked chicken and potatoes.
Let the curry simmer for about 5 minutes. If it gets too thick add a bit more curry base. Don't add water.
Garnish with a bit of chopped fresh cilantro if you like. Serve with rice or Indian flatbread (or both!).