1. Add the cumin,coriander,fennel and celery seeds to a mortar and grind roughly. Add the ground spices and grind to combine.
2. Finely chop the chicken by hand.
3. Heat the oil for one minute in a saucepan or deep-sided frying pan over medium heat. Add the onion,garlic and celery and cook for six to eight minutes while stirring frequently. Add the chilli and cook for two minutes. Add the ground spices and fry for 30 seconds. Add the chicken and cook for five minutes. Add the bay leaf,sugar,tomato paste and cocoa powder. Season lightly with salt. Add the vinegar and chicken stock and bring to a simmer. Reduce the heat to low and cook for 25 to 30 minutes,stirring often. Cook until the sauce is quite thick and starts to glaze. Adjust the seasoning.
4. Serve with coriander leaves,soft tortillas,creme fraiche or sour cream,guacamole,charred corn,chilli, kale slaw or anything else that takes your fancy.
TIPS
1. You can adjust the spice mix to your taste and dial up the heat with extra dried or fresh chillies if that's your thing.
2. Be careful not to season too heavily (and don't use stock with salt or sugar in the ingredients),as the sauce reduces a lot. Just correct at the end.
3. This freezes well,so a big batch like this is great to have on hand when you're short on time.