Preheat oven to 350F (175C) and lightly grease an 8x8 glass baking dish².
In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt.
Make a well in the center of the bowl and add the water, oil, vinegar, and vanilla extract.
Whisk or stir all ingredients together until batter is smooth.
Pour batter into prepared baking dish and bake on 350F (175C) for about 35 minutes or until a toothpick inserted in the center comes out clean. Do not over-bake or the cake will be dry.
Allow cake to cool before frosting.
To prepare the icing, whick together sugar, cocoa powder, water, vanilla extract, and salt until smooth. Icing should be a thick and fudgy consistency, but should be spread-able. If it is too thick, add additional water one teaspoon at a time until the proper consistency is achieved.
Once cake has cooled, gently spread frosting evenly overtop. Enjoy!
¹You may substitute apple cider vinegar, but beyond that I do not know of an acceptable substitute for the vinegar in this recipe. Please see the post for the role vinegar plays in this recipe.²If you bake this in an aluminum baking dish it might cause the cake to have a metallic taste. If you only have this type of dish, you can line the pan with parchment paper to prevent this.This cake may instead be baked in a 9x9 or 11x7 baking dish, but keep an eye on it as the baking time will be reduced.You can make this a vanilla cake by leaving out the cocoa powder.STORING: Store covered at room temperature for up to 5 days. Cake will keep even longer if stored in an airtight container in the refrigerator by the fridge may dry out the cake a bit.