Mix together the onions, coffee, tomato sauce, Worcestershire sauce, brown sugar, cider vinegar, hot chillies, chilli powder, garlic and salt in a saucepan until the brown sugar has dissolved. Place over medium heat, bring to a boil, reduce heat and simmer for 30 minutes.
Pour the sauce into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel and carefully start the blender, using a few quick pulses to get the sauce moving before leaving it on to puree. Puree in batches until smooth. Store in glass container in refrigerator.