Rinse the rhubarb and slice into 1-inch pieces, discarding any leaves. Place in a heavy-bottomed pot with sugar, water, and lemon juice. Heat over medium heat, stirring frequently, until the rhubarb releases its juices and comes to a boil. Reduce heat and simmer for 15 to 20 minutes until completely soft and broken down.

Carefully transfer the cooked rhubarb to a blender or use an immersion blender directly in the pot. Blend until completely smooth. Be careful when blending hot liquids: secure the lid, vent it, place a towel over the top, and stand back.

Return the puree to the pot and bring to a gentle simmer over medium-low heat. Cook, stirring frequently, until thickened and reduced, about 15 to 20 minutes more. The butter will darken slightly and become glossy. To test, drag a spoon across the bottom of the pot. If the butter holds its shape and doesn't immediately fill in the gap, it's ready.

Ladle the hot butter into prepared jars, leaving 1/4 inch headspace. Remove air bubbles, wipe rims clean, and apply lids and bands to fingertip tight.

Lower filled jars into a prepared water bath canner, ensuring they're covered by at least 1 inch of water. Process for 10 minutes for half pints or pints, adjusting for altitude.

Turn off heat and let jars stand in canner for 5 minutes before removing. Place on a towel-lined counter and cool undisturbed for 12 to 24 hours. Check seals before storing. Refrigerate any unsealed jars for immediate use.