• Sort the blueberries removing any bad berries, stems, and flowers. Then rinse with cool water to clean.
• Puree the blueberries in a blender or food processor.
• Pour the blueberries into the slow cooker and add the sugar, lemon juice, and cinnamon. Stir well.
• Turn the slow cooker on to low and cook for 1 hour.
• After the first hour, prop the lid of the slow cooker open with a wooden spoon to let the liquid evaporate. Continue cooking on low for 4 to 5 hours. Check every hour to see how thick it is.
• It’s done when the butter mounds on a spoon and doesn’t run off.
• Ladle the butter into half-pint jars, leaving 1/4 inch headspace. Process in a water bath canner for 10 minutes.

To Can:

• Ladle the butter into half-pint or pint jars, leaving 1/4 inch headspace.
• Remove air bubbles with a debubbler, then wipe the rims clean.
• Add lids and rings, tightening finger-tight, then place jars in a water bath canner.
• Process for 10 minutes. Then turn off the heat and let the canner sit for 5 minutes to cool down.
• Remove the jars from the canner and place them on a towel. Let them sit undisturbed for 12 to 24 hours. Then check the seals. Any jars that didn't seal, place them in your fridge to use up first. Then wash, dry, and label the other jars for storage.

To Freeze:

• Ladle the butter into freezer-safe containers.
• Let it cool fully, then freeze for up to a year.
• Thaw in the fridge before using.