Place a small plate in the freezer before you start cooking.

To a 3-quart or larger saucepan (the larger the surface area, the shorter the cooking time), add the rhubarb, sugar and lemon juice.

Bring to a boil over medium-high heat while stirring frequently. Turn the heat to the lowest you can to keep it at a full boil. Let boil for about 9 minutes, turning the heat down, if needed, towards the end to prevent the rhubarb from burning, until it reaches a temperature of 220°F (104°C). This is the temperature jam is said to set, but I couldn’t actually get mine above 212°F (100°C). It’ll still be a bit runny but will thicken as it cools and more as it’s chilled.

When you think the jam is ready, drop a small spoonful onto the cold plate and let it sit for a minute. Push it with your finger - if it wrinkles and holds its shape, the jam is done.

When it’s done, stir in the vanilla extract.

Remove the saucepan to a wire rack to cool completely, about 1 hour.

Transfer into clean jars and refrigerate for up to 1 week. Many sources say 2-3 weeks, but I like to play it safe and freeze whatever I haven't eaten after 1 week. It also freezes great and can be frozen for up to 1 year.