My local shops sell a Tunisian brand of harissa called Le Phare du Cap Bon. It is good, and has a kick -- though less of one, I was interested to discover, in France, to which the Tunisians appear to export a milder version. But it lacks the zing of a home-made sauce.
The small dried chillis that are widely available are not ideal for the job. They are viciously hot; even I, a lover of hot things, find that they blowtorch flavour out of the food. The sauce should be fiery, but with a heady spiciness.
I got a tube of Bart aji amarillo chillis in my Christmas stocking. They are very hot too: next time I use them, I shall remove the membrane and seeds. But they make a more flavoursome harissa.
1 Bart aji amarillo chilli
1/2 tsp caraway seeds
1/2 tsp cumin seeds
1 clove garlic
Pour boiling water over the chilli, and leave to soak for half an hour or longer. (It has to be soft before it can be worked into a paste.) Meanwhile, put the caraway and cumin into a dry saucepan, and cook over a gentle heat until toasted.
Cut the chilli into pieces, and put it into an electric herb mill or coffee grinder -- if you don't have either, you'll need a pestle and mortar -- with the caraway, cumin, garlic (which you might want to chop a bit first), and a drop or two of olive oil. Whizz. You'll probably need to scrape the sides of the mill, and whizz again. Add a little salt. Keep adding drops of oil until the mixture starts to form a smooth paste.
Most harissa recipes include caraway. Some include coriander instead of, or in addition to, the cumin.
Store the harissa, with a layer of oil covering it, in a small jar in the fridge.