This twice-baked Greek bread is known for its durability, keeping for months.  The trick seems to be to know how much to soften it with water when you want to use it. Here is one recipe, and there are others of a sweet version, many of which have a strong resemblance to the Italian cantuccini. Happy baking! 

‘Reaching into the hollow of the log, he extracted three paximadia from the spring and wrapped them in a cloth to draw the water out before they got too soggy. These dark brown pumices of twice-baked bread – the staple fare of Greek shepherds and of the medieval Basilian hermits – can be kept for months.  Hard as fossils, they are excellent; especially with garlic, when soaked to the right consistency.’ 

Source: Patrick Leigh Fermor, Mani: Travels in the Southern Peloponnese, introduction by Michael Gorra, New York: New York Review of Books, 2006 (1958), p. 24