Preheat the oven to 400 degrees F. Line a standard muffin pan with liners and spray them lightly with oil.
Combine the soy milk and apple cider vinegar in a glass measuring cup or small bowl and set aside to curdle. This is your vegan buttermilk.
In a large bowl, whisk together all the dry ingredients: flour, cocoa powder, granulated sugar, baking powder, salt and cornstarch. If the cocoa powder is really clumpy, sift it in.
Pour the vegan buttermilk, oil and vanilla into the dry ingredients bowl. Stir with a spatula gently, being careful not to over mix. Over mixing muffin batter makes them hard instead of soft.
Fold in most of the chocolate chips, saving about 1/4 cup for the tops.
Fill muffin cups nearly to the top and sprinkle a few more chocolate chips on the tops.
Bake for 26-28 minutes, until a toothpick inserted in the middle comes out mostly clean. Be careful not to over bake, or the muffins will become dense and dry.
Let them cool for 5 minutes in the pan, then transfer to a cooling rack.
Enjoy! Store leftover muffins in an airtight container at room temperature for about 3 days. They also freeze perfectly.