Peel and chop: Using a potato peeler, or sharp paring knife, peel the guavas making sure to only remove the thin skin covering it. Discard the skins.Cut the guavas in halves and scoop out the seeds with a teaspoon. Reserve the pulp for use in a later step.
Boil seeds: In a saucepan, add the seeds, cinnamon, cloves, and anise plus 6 cups of water.Boil for ten minutes over medium heat, or until the seeds separate, and the water turns a caramel color.Strain the liquid and discard the seeds, but extract as much as you can of the mass between the seeds. Return the liquid to the pot.
Boil the pulp: Add the peeled guavas you had saved to the pot with the liquid, and add sugar, plums, and vanilla, and heat over low heat. Boil until the guava halves are soft, adding water if necessary to keep them from drying out.Once the pulp is soft, and you obtain a slightly thick syrup, remove it from heat and cool to room temperature.
Serve: Chill in the refrigerator before serving.
Notes
If you are watching your weight, add less sugar and just enjoy the natural sweetness of this delicious fruit.Prunes are not part of the traditional recipe, they are my addition, I just like them in this dish, but you can leave them out if you wish.Another way in which I have seen these shells served is with diced raw "queso de freír", if you don't have any, halloumi makes an acceptable substitute.