Begin by preheating your oven to 350 degrees Fahrenheit. Lightly grease (with butter) AND line with parchment paper the bottoms of THREE 6-inch cake pans or TWO 8-inch pans.
In a bowl or measuring cup, add the milk and apple cider vinegar together, whisking until combined. Let it sit for 5-10 minutes until the mixture slightly thickens.
In another bowl, add together the espresso powder and boiling water, mixing until powder fully dissolves.
In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, cinnamon, and salt until combined.
Add in the milk-ACV mixture, melted butter, melted chocolate, and espresso mixture and mix together using a rubber spatula or wooden spoon, until just combined.
Divide the batter evenly into each of the cake pans and bake for 20-25 minutes, or until a toothpick or butter knife comes out clean when tested in the centers.
Bake for 30-35 minutes, until the center of the cakes come out clean using a toothpick.
Remove cakes from the oven and let them rest in the pan for 10 minutes or so before removing the cakes and placing them onto a cooling rack to fully cool. NOTE: Let them cool for at least 1-2 hours in a cool place.