Preheat oven to 400 degrees F.
Place plums evenly in a deep dish 9’inch pie plate. Add the maple syrup, lemon juice, and vanilla bean and stir well. Place reserved pits and cardamom pods together in a corner of the plate and let sit for at least 30 minutes (or covered overnight in the fridge).
Put the pie plate on a baking sheet and place in the oven. Bake stirring the mixture occasionally for an hour.
Prepare a glass jar and lid by washing in very hot water then placing upside down to dry on a clean kitchen towel.
When the liquid is syrupy and has reduced and the plums have broken down to a jammy consistency, remove from the oven and fish out the pits and cardamom pods.
Using a canning funnel, fill prepared jar with the jam, screw on top and let cool completely.
Place jam in fridge and eat within one month for best flavor.
