Rinse cherries, use a cherry pitter to remove pits, then transfer to a medium saucepan.
Add the honey and simmer gently over low heat, uncovered. Once the honey has melted, stir in the lemon juice and sea salt.
Simmer for 5 minutes. Transfer 2 tablespoons of the hot cherry liquid to a small bowl with the cocoa powder. Whisk until smooth, then stir the cocoa mixture back into the saucepan along with the vanilla extract.
Continue simmering the cherries for another 5 minutes, stirring occasionally. Stir in the chia seeds and cook for 1–2 minutes, stirring frequently. Remove from the heat. The jam will continue to thicken as it cools.
Transfer the jam to clean, sterilized jars, leaving enough room for expansion if freezing. Allow to cool, then cover, label, and refrigerate.
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Notes
If you're unsure whether the jam is thick enough, refrigerate a small amount until cool to check the consistency. If it's still too thin, stir in additional chia seeds and simmer for one more minute before ladling the jam into jars.
Sour cherries can vary quite a bit in tartness, so start with less honey and add more if needed.
Chia jam thickens considerably as it cools, so don't add extra chia seeds while the mixture is still hot and runny.
It may seem insignificant, but a small pinch of salt enhances both the cherries and cocoa and keeps the jam from tasting flat.
Because this recipe isn't formulated and tested for water-bath canning, don't process it as a shelf-stable canned jam.