• Add all ingredients to a large saucepan and bring to a boil over medium-high heat. Reduce heat to medium-low and simmer until soft, about 15-20 minutes.
• Puree the contents of the pot with an immersion blender (or carefully transfer to a blender and blend, then return to the pot) until smooth.
• Reduce heat all the way to low and continue cooking, stirring occasionally, for 1-1/2 to 2 hours, or until the rhubarb butter is thickened (see photos in recipe for a guide) and doesn't run when a bit is added to a plate.
• Decide on your storage: refrigerate and eat within a month, or freeze or water-bath can for longer storage.

To Can:

• As rhubarb butter nears completion, prepare three 1/2-pint jars and keep hot until needed. Prepare canning lids according to manufacturer’s directions, and start the water heating in a canner.
• When the butter is ready, ladle into 1 jar at a time, leaving 1/4-inch headspace, wiping the rim with a damp cloth, and attaching the lid. Place jar in prepared canner. Fill and close remaining jars.
• Process jars in a boiling-water canner for 15 minutes. Remove lid, turn off heat and let jars cool in canner 5 minutes before transferring to a towel-lined counter to cool. Check seals before labeling and storing in a cool, dark place.

To Freeze:

• Let rhubarb butter cool a bit and divide into freezer-safe jars, leaving 2-inches headspace for expansion. Seal, label and freeze for up to a year.
• Thaw before using and keep in the refrigerator (it will keep in the fridge for a few weeks).