connrrr I have been adding coffee and high % dark chocolate to my japanese curries for a while now. I hadn’t considered soy sauce or miso paste however, and will certainly try either or both of those next time. I also hadn’t considered fully caramelising the onions to start with, and I’m quite interested to see how that turns out.
I usually only use 1 or 2 teaspoons of instant coffee for a whole batch (several litres) worth of curry, where as a drink a coffee cup is probably more around 200ml or so. The caffeine will be highly diluted so if it is the effects of caffeine you are concerned about then it may not be an issue. If you are trying to remain 100% abstinent however, then of course it can be excluded. In the case where you omit coffee powder entirely, perhaps ½ - 1 teaspoon of cocoa powder to go along with the dark chocolate may help.
Another Secret Ingredient that works rather well in anything where you are aiming for an umami based profile is star anise. One star anise in with any sort of stewed or simmered sauce like this (you’ll see I use one in my goulash recipe) can really add to that umami flavouring – and does not add any anise style characteristics to worry about.