Add warm water to a heavy-duty pot or heat-safe container, attach your immersion cooker and set to 90ºC/194ºF. Add the empty, capped jars to the water to warm up - this will prevent the glass from cracking or breakage during cooking. Cover the pot/container to speed up heating.
Make the jam:
Wash rhubarb, remove and discard any rough edges. Slice the rhubarb into thin, even pieces and set aside.
Add sugar, pectin and salt to a medium-size bowl and whisk to combine well. Add the chopped rhubarb, vanilla beans and cordial. Toss well to coat and allow to rest for 10-15 minutes.
Remove jars from heated water bath, unscrew caps with a towel or oven mitt and place on a towel to avoid thermal shock. Spoon rhubarb mixture into the two jars and close the jars enough to easily open with only your finger tips. This will allow trapped air to release, but prevent water from entering and the glass from cracking.
Use a jar-lifter or tongs to add filled jars to the heated water bath. Cook for 30 minutes. When ready, remove jars from the water bath and place on a towel to cool at room temperature. Once cooled, refrigerate to set overnight.
When ready to use, remove from refrigerator, stir and enjoy! I especially like it swirled into yogurt or ice cream.