Preheat the oven to 350 degrees. Grease a 13x9x2 baking dish and dust it with flour
In a large mixing bowl, sift together the flour, cocoa powder, sugar, salt and baking soda. Use a whisk to thoroughly combine the ingredients.
In a separate medium bowl, add the water, vanilla extract and vegetable oil. Stir to combine.
Now add the wet ingredients to the bowl of dry ingredients and whisk until smooth.
Add the apple cider vinegar and whisk just until it is incorporated.
If using the mini chocolate chips, add them now and gently stir just to mix the mini chocolate chips throughout the batter.
Pour the batter into the prepared pan and bake for 50 to 55 minutes or until a knife inserted in the middle comes out clean. (Do not underbake this cake. If the middle is slightly undercooked, the cake will sink in the middle as it cools.)
Let the cake cool.
After the cake has cooled, make the glaze. Put the chocolate and boiling water in a blender and allow it to set for a couple of minutes so the chocolate can soften. Now turn on the blender to blend just until smooth. Pour the glaze in a bowl and cool for 15 to 20 minutes, so the glaze can be at a spreadable consistency. Spread the glaze over the top of the cake.
The cake is ready to be cut and served.