Wash the rhubarb stalks thoroughly and trim the ends. Cut into 1/2-inch pieces.

In a large, heavy-bottomed pot, combine the chopped rhubarb, granulated sugar, and lemon juice.

Stir the mixture well and let it sit for about 30 minutes to allow the rhubarb to release its juices.

Place the pot over medium heat and bring the mixture to a boil, stirring frequently to prevent sticking.

Reduce heat slightly and continue to simmer for 15-20 minutes, or until the jam has thickened and reached 220°F (104°C).

Remove from heat and stir in the vanilla extract if using. Skim off any foam from the surface.

Ladle the hot jam into sterilized jars, leaving 1/4 inch of headspace, and seal tightly.