Preheat the oven to 350°F (175°C). Grease and line two 8-inch (2x20cm) cake pans.
Stir the apple cider vinegar into the non-dairy milk and set aside to curdle into vegan buttermilk.
Sift the flour, cocoa powder, sugar, baking powder, baking soda, and salt into a bowl and mix to combine.
Stir the coffee granules, if using, and vanilla extract into the boiling water.
Pour the vegan buttermilk, oil and hot water mixture over the flour mixture. Also, add the apple sauce and stir with a spatula or handheld mixer on low speed just until combined and no large lumps remain (please do not overmix). (Adjust sugar to your desired taste).
Fill the batter into the prepared cake pans and bake for about 40-45 minutes, or until a skewer inserted into the center of the cake comes out with just a few moist crumbs attached. (If you want a moist chocolate cake, make sure not to overbake it.)
Remove from oven and let cool completely. (Optionally, you can place the cakes in the fridge for at least 2 hours or overnight, as they will be easier to work with when well chilled.)