Cut the unpeeled apples into large chunks, then remove and compost the cores and any bruised bits.
Tip the diced apples into a heavy-based pan with the apple cider vinegar, vanilla extract, sea salt, cinnamon (or apple pie spice mix) and 180ml water.
Bring to a boil, then turn down the heat to a strong simmer, half cover the pan and cook, stirring occasionally, for about 40 minutes, until reduced by half.
Take off the lid, turn up the heat a touch, add the maple syrup and reduce for 20 minutes more, until the mixture turns thick and dark brown, and holds its shape when you stir it.
Pour into a clean 500ml jar, seal and store in the fridge for up to a month, or freeze in smaller portions for up to a year.
