Preheat the oven to 375F and prepare a loaf pan with parchment or spray oil.
In a small bowl, combine the ground flaxseed with water to make flax egg. Allow that to sit for 15 minutes.
In the meantime, combine the wet ingredients, including the milk, apple cider vinegar, vanilla extract and coconut yogurt. Add in the flax egg and stir.
In a separate bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda and salt.
Pour the wet ingredients into the dry and gently stir until a batter is formed. Fold in the dark chocolate chips.
Transfer to the loaf pan, and bake for 30 to 35 minutes, until the center is set. Remove from the oven and allow to sit for at least 10 minutes before slicing. Store on the countertop in a sealed container for up to five days.