Pit cherries with either a hand pitter, or using the simmer method described above.

Once pitted, puree cherries smooth. Place the puree in a slow cooker and cook on low for 8 to 16 hours, until the cherry puree has reduced by half and is quite thick.

Puree the mixture again with an immersion blender until very smooth. Add sugar to taste and stir until it is completely distributed and dissolved.

Funnel the finished butter into 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 15 minutes.

When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.