Place chopped pears, apple cider and lemon juice in a 4-6 qt slow cooker.
Set the heat on high and simmer, covered, until pears have softened, about 40 minutes. Turn heat off.
Process pear mixture using an immersion blender, or in batches in the bowl of a food processor, until puree is smooth.
Combine pear mixture with brown sugar and salt in the slow cooker.
Turn heat to high and place lid on a diagonal over slow cooker, allowing steam to escape at both ends. Simmer on high, stirring every 30 minutes or so, until puree thickens, darkens in color, and holds it’s shape on a spoon, about 3-4 hours.
Prepare boiling water canner. Heat jars in simmering water until ready to use. Do not boil. Wash lids in warm soapy water and set aside with bands.
Ladle hot pear butter into a hot jar leaving a 1/4 inch headspace. Remove air bubbles. Wipe jar rim. Center lid on jar and apply band, adjusting to fingertip tight. Place jar in boiling water canner. Repeat until all jars are filled.
Process jars 15 minutes, adjusting for altitude. Turn off heat.
Remove lid and let jars stand 5 minutes. Remove jars and cool. Check lids for seal after 12 hours. Lid should not flex when center is pressed.