01 -
Trim the rhubarb ends, discard all leaves, rinse the stalks well, and cut them into 1-inch pieces.
02 -
Add rhubarb, sugar, lemon juice, water, and salt to a wide heavy-bottomed pot. Cook over medium heat until bubbling, then reduce to medium-low and simmer for 12-15 minutes until the rhubarb breaks down.
03 -
Remove the pot from the heat. Blend with an immersion blender until smooth, or carefully transfer the hot mixture to a blender in batches.
04 -
Return the puree to medium-low heat. Simmer for 20-25 minutes, stirring often, until glossy and thick enough that a spoon leaves a brief trail across the bottom of the pot.
05 -
Stir in the vanilla. Taste and add a little more sugar for sweetness or lemon juice for brightness if needed.
06 -
Spoon the rhubarb butter into clean jars. Cool, cover, and refrigerate for up to 2 weeks or freeze for up to 6 months.