Pit and halve plums. Juice lemon. Add plums, lemon juice, sugar, cinnamon, and vanilla paste to a saucepan, and stir to combine. Let sit for at least 1 hr.
Purée everything with an immersion blender, then bring to a boil while constantly stirring. Let simmer for approx. 5 min.
To test doneness, spoon hot jam on a cold plate. If the jam feels gel-like, then it's ready. Fill hot jam into glass jars with with a measuring cup and seal. The jam will last up to a few months. Enjoy!