Peel pears and cut out stem and cores. Cut into large chunks.
Combine pears, cranberries, water, lemon juice and cinnamon stick in a large non-stick pot. Cover and bring to a boil over high heat, stirring occasionally. Reduce heat to low and simmer covered for 1 hour. Stir occasionally. Pears should be very soft.
Allow to cool slightly and, remove cinnamon stick and set it aside. Transfer mixture to a blender or use an immersion blender in the pot to purée.
Put 2 small saucers in the freezer so you can eventually check the thickness of the butter.
Return mixture and cinnamon stick to a clean non-stick pot. Add the sugar and salt. Bring to a boil over medium high heat, stirring often to dissolve sugar. Reduce heat and simmer steadily, stirring often for about an hour until mixture thickens. Keep cleaning edges of the pot so you don't get a dried rubbery ring of fruit.
To test for doneness, spoon out a teaspoon of the butter to the chilled plate and freeze for 1 minute. Butter should hold its shape and not spread. Continue to reduce until it passes this test.
While the butter is reducing, sterilize 4 half pint jars or 8 1/2 cup jars by inverting them in boiling waters for 15 minutes. Sterilize lids in boiling water for 5 minutes just before you are ready to seal the jars.
When butter is thick enough pour into the prepared jars. Stir to remove any air bubbles, wipe the rims and place the lid on the jar. Put on the screw band and turn until finger-tip tight. Allow jars to cool at room temperature. Label and store in the fridge for up to 1 month.