Put the plums, apple cider vinegar, brown sugar, garlic, dark soy sauce, ginger, chili, cloves, allspice, black pepper and salt in a large heavy-based saucepan.
Bring the mixture to a gentle boil over medium-high heat, stirring until the sugar has dissolved.
Reduce the heat to medium-low and simmer uncovered for about 30 minutes, stirring occasionally, until the plums have completely softened and collapsed.
Remove the pan from the heat. Blend the sauce with an immersion blender until smooth.
If using a countertop blender, let the mixture cool for about 10 minutes. Blend it in small batches with the center cap removed and the opening covered loosely with a folded kitchen towel so steam can escape.
Return the sauce to the saucepan and simmer over low heat for 10 to 15 minutes, stirring often, until it coats the back of a spoon.
Cool a small spoonful and check the consistency. Continue simmering if the cooled sauce is still too loose.
Remove the pan from the heat and let the sauce cool slightly before transferring it to clean jars or freezer containers.
Refrigerate once cool. The sauce will set further as it chills.
