Coarsely chop the rhubarb. The mixture will cook down and become homogeneous, even with larger chunks.
In a heavy-bottomed kettle, combine rhubarb, sugar, zest, juice and water. Place over medium-high heat. Bring to a rolling boil (one that cannot be stirred down).
Once rolling, bring the boil down slightly and continue cooking for 20 minutes. The rhubarb will completely break down and the butter will become thick.
After 20 minutes, remove from heat and pour into a conical strainer(affiliate link) or food mill(affiliate link). Press through, ensuring to extract all possible liquid until left with a dry and gummy paste (this contains all the rhubarb strings and skins).
Pour hot butter into sterilized jam jars and place lids and rings on top.
Set directly into a boiling water bath, ensuring jars are covered with one inch of water. Bring back to boil and process for 10 minutes.
Remove from water bath with jar lifter and let rest until completely cool. Be sure to listen for that “ping” as the lids seal.
The variety of rhubarb does not matter; use whatever your garden or local market has. If the rhubarb is a green variety, you may want to color the end result with red food coloring.