In a large, heavy pot, combine lemon zest and juice with apples. Stir well to coat.

Stir in sugar, cocoa powder, and salt. Bring to a boil over medium heat, stirring constantly. Reduce heat and simmer, stirring frequently, for 20–30 minutes or until apples are soft. Using a blender and working in batches, puree apple mixture until smooth.

Meanwhile, heat six 1-cup jars and lids.

Return puree to a clean, large pot. Add vanilla and bring to a gentle boil over medium heat, stirring constantly. Cook, stirring constantly, about 4–10 minutes, or until mixture thickens enough to mound on a wooden spoon. Fill hot jars, leaving 1/4-inch headspace. Top with lids. Process jars at a full rolling boil for 15 minutes.