Preheat oven to 350°F (175°C) and grease a 9-inch round cake pan or line with parchment paper.
In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, and salt.
Add wet ingredients: vegetable oil and non-dairy milk (plus vanilla extract if using), and stir until smooth and well combined.
Add apple cider vinegar if desired, and gently fold it into the batter; it helps the cake rise.
Pour batter into the prepared pan, spreading evenly.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Optional: Frost with vegan chocolate ganache or dust with powdered sugar before serving.