Place plates in the freezer for use in gel test. Wash clementines and place in the refrigerator for at least 30 minutes. Chilling the clementines will make it easier to slice them.
Using mandoline, slice seven of the clementines on the 1/8" setting. Remove any seeds and place slices in a large saucepan. Add cold water to cover and bring to a boil over high heat. Reduce heat to medium and simmer for ten minutes. Drain clementines and discard the cooking liquid.
Once the simmered clementines are cool enough to handle, roughly chop them using a knife or food processor until they are the desired texture. Weigh the fruit and return the chopped clementines to the saucepan. Add sugar to the saucepan equal to the weight of the fruit. I like to mix white and brown sugar at a 50/50 ratio to achieve the total amount of sugar required.
Prepare the canning pot with a rack in the bottom by filling with water and bringing it to a boil. Sterilize the half-pint canning jars and lids and keep warm until ready to use.
Juice the remaining three clementines. If necessary, add water to the juice in order to equal one-half cup of liquid. Add this liquid to the saucepan and stir to combine. Bring the mixture to a boil over high heat. Reduce heat to medium and boil until the mixture passes the gel test when allowed to cool on the chilled plates set aside in the freezer. This process will take between 15 and 30 minutes depending on the amount of liquid in your mixture.
Once the mixture has passed the gel test, remove the marmalade from the heat and stir in the vanilla. Ladle hot marmalade into sterilized half-pint jars. Remove air bubbles from the side of the jar. Using a clean cloth, remove any residue from the rim of the jar. Place a lid on the jar and tighten with a band. Gently lower the filled jar into the boiling water canner. Repeat until all jars have been added to the pot. Place lid on canning pot. Return water to a boil.
Process half-pint jars of marmalade for 15 minutes. Turn off heat and remove the lid from the pot. Allow the jars to sit undisturbed for five minutes. Carefully remove jars to a towel-lined baking sheet. Allow jars to cool up to 24 hours before checking the seals and labeling the jars for storage.
Notes
Properly sealed clementine marmalade can be stored up to one year.I can't take credit for this recipe. I found it by pure luck on the Seasons and Suppers site. It is perfectly written and yields me predictably delicious results.