Bulgarian Plum Jam (Marmalad ot Slivi)

Steps

1. 1
   Wash and stone the plums and quarter them, keeping the skins on: they give the jam its colour and help it set.
2. 2
   Put them in a wide, heavy pot over low heat, cover and cook for 30 minutes, stirring now and then, until the plums collapse into their own juice.
3. 3
   Push the plums through a food mill, or blend them with a stick blender, for a smooth jam; leave them coarse if you prefer.
4. 4
   Add the sugar and cinnamon stick and cook uncovered over low heat for 2–3 hours, stirring often with a long wooden spoon and scraping the base. It spits as it thickens, so protect your hand.
5. 5
   It is ready when it is dark and very thick, a spoonful on a cold plate does not run, and the spoon leaves a clear path across the bottom of the pot.
6. 6
   Remove the cinnamon stick, fill hot sterilised jars, seal and turn them upside down for 10 minutes. Stored cool and dark, it keeps for well over a year.