from Food in Jars
makes approximately 3 1/2 pints of butter
8 cups of pureed blueberries
Put the pureed blueberries in a slow cooker. Place a lid on the pot and turn it on low. After about an hour, give it a stir. At this point, you want to use something to prop the lid a bit. I found that laying a wooden spoon across the rim of the cooker and then placing the lid on gave it just enough room to let the steam evaporate.
After 5 hours add spices and sugar. Remove the lid and increase heat setting to high.
it’s cooked down sufficiently*, pour into jars (leave a good 1/2 inch
of head space), wipe rims, apply lids and screw on bands. Process in a
boiling water canner for 10 minutes.
*When the cooking process is done, you can puree with an immersion blender or (carefully) in a regular blender, for a smoother product. It depends on whether you like your butters a bit chunky or very smooth.