Skin the tomatoes by making a criss-cross on the bottom of each one and then plunging them into a pan of boiling water for a couple of minutes. Remove with a slotted spoon and transfer to refresh in a bowl of ice cold water. Once cool, carefully remove the skin.
Scoop out the seeds of the tomatoes over a sieve to collect the juices, and tear the flesh into small pieces. Set aside in a bowl.
In a very hot pan, place the onion, cut side down with the unpeeled garlic cloves and cook until charred. Turn onto the other side for a few minutes, and then remove and peel both the garlic and the onion, then chop finely. Set aside the garlic and onion separately.
Dry fry the chilies in a hot pan for about 4-5 minutes, turning frequently so that they don’t burn – they should lighten in colour when they are done.
Add the chillies to a bowl with the cola and a splash of water and leave to plump for about 15 minutes.
In a saucepan, gently fry the chopped onion in the sunflower oil with the oregano, coriander stalks, black pepper and a splash of water until the onions are soft, but not coloured.
Add the garlic, tomatoes and their juice, cloves and ground cumin and simmer gently for about 10 minutes. Season with salt.
Add the orange juice, the vinegar and the rehydrated chilies to the pan and, using a stick-blender, blend until smooth.
Pass through a sieve into a clean saucepan and bring to the boil. Cook gently for about 30 minutes, then add the cocoa powder and cook for a further 10-15 minutes, or until the mixture is the consistency of a pouring sauce.
Add more cola, orange juice, seasoning and spices to taste if necessary.
Pour into sterilised bottles and seal.