Prepare the Rhubarb: In a large, non-reactive saucepan (like stainless steel or enamel), combine the chopped rhubarb, granulated sugar, and water. Stir gently to coat the rhubarb.

Macerate and Initial Cook: Let the mixture sit at room temperature for about 15 minutes to allow the sugar to draw out some liquid from the rhubarb. Place the saucepan over medium heat.

Simmer and Soften: Bring the mixture to a low boil, stirring occasionally. Once boiling, reduce heat to low and simmer, uncovered, for about 20-25 minutes, or until the rhubarb has completely broken down into a sauce consistency.

Achieve Set Point: Stir in the lemon juice, lemon zest, and vanilla extract (if using). Increase the heat slightly to medium-low and continue to simmer gently, stirring frequently to prevent scorching. Cook for another 15-20 minutes. The jam is ready when it thickens and coats the back of a spoon, or reaches 220°F (104°C) on a candy thermometer.

Cool and Store: Remove the jam from the heat. Allow it to cool for about 10 minutes, stirring occasionally, as this further aids in the natural setting process. Skim off any surface foam if desired.

Jarring: Ladle the hot jam into sterilized jars, leaving 1/4-inch headspace. Wipe rims clean, apply lids, and process in a boiling water bath for 10 minutes for shelf-stable storage, or refrigerate immediately for non-processed jam.