Prepare the fruit: In a medium, heavy-bottomed saucepan, combine the chopped rhubarb, granulated sugar, and lemon juice. Stir gently to combine.

Macerate: Let the mixture sit at room temperature for at least 30 minutes (or up to 2 hours in the refrigerator). This allows the sugar to draw out the juices from the rhubarb.

Cook the jam: Place the saucepan over medium-high heat. Bring the mixture to a full, rolling boil, stirring frequently to prevent scorching on the bottom.

Thicken the jam: Once boiling, reduce the heat slightly to maintain a vigorous boil. Stir constantly for about 15-20 minutes, or until the jam reaches 220°F (104°C) on a candy thermometer, or when a small spoonful thickens quickly on a chilled plate.

Finish and jar: Remove the pan from the heat. If using, stir in the vanilla extract. Skim any foam from the surface. Carefully ladle the hot jam into sterilized jars, leaving 1/4 inch headspace. Wipe the rims clean.

Process or cool: For immediate consumption, allow the jars to cool on the counter and refrigerate. For long-term storage, process the sealed jars in a boiling water bath for 10 minutes, adjusting time for altitude.