Preheat oven to 350°F. Lightly grease an 8″x11″ baking pan (or similar sized baking pan, such as a 9″/10"-square or springform or loaf pan *read my blogpost for details). Then line with parchment paper, cut to fit if needed (*as pictured in the step-by-step pics).
In a jar, stir the apple cider vinegar into the non-dairy milk and set aside for about 5-10 minutes to create vegan buttermilk.
Using a sifter (or a fine sieve), sift the flour, sugar, cocoa powder, baking soda, baking powder, and salt into a large mixing bowl. Whisk to combine, then set aside.
Melt the coconut oil and 1/2 cup of the chopped chocolate over a water bath (or microwave). Add the vanilla extract and stir to combine.
Pour the vegan buttermilk and melted chocolate mixture over the flour mixture and stir until just combined and no large lumps remain (please do not overmix or the cake will get dense). Then, fold in the remaining chopped chocolate. (Taste and add more sugar if desired).
Fill the batter into your prepared baking pan and bake for approx. 30-35 minutes or until a toothpick inserted comes out almost clean with a few crumbs attached (*see recipe notes). Allow the cake to cool completely before adding the frosting.