Wash plums, cut them in half, and remove pits.

Place the plums into the slow cooker. Add 2 tablespoons of water and put the lid on the cooker.

Set the cooker to high and cook for 2 to 4 hours, until the plums are tender and have released a great deal of liquid.

Using an immersion blender, puree the fruit until smooth.

Set a wooden chopstick across the rim of the slow cooker and set the lid on so that it is vented slightly.

Turn the cooker on to low heat and cook the fruit puree for 6 to 10 hours, until the plum puree has reduced into a thick, dense, spreadable paste.

Scrape the finished butter into a medium saucepan and puree again with your immersion blender. Place on the stove over low heat and stir.

Add sugar, spices, and lemon juice. Taste and adjust as necessary. Keep stirring until the butter is hot and the sugar and spices are fully incorporated.

To process, funnel the finished butter in half pint jars, leaving 1/2 inch headspace.

Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 20 minutes.

When time is up, turn off the heat, remove the lid from the canner, and allow the jars to cool gradually in the water.

After they've slowly cooled for 6 to 7 minutes, remove the jars from the canner and set them on a folded kitchen towel to cool.