Preheat oven to 375º F and lightly grease two 9-inch cake pans with butter or non-stick spray.
Whisk together flour, sugar, cocoa powder, baking soda and salt in a large bowl, then whisk in water, oil, cider vinegar, coffee and vanilla extract.
Stir until no lumps remain, then pour mixture evenly into greased cake pans.
Place in oven and bake for 24-26 minutes, or until toothpick inserted in center comes out clean.
While cake bakes, make frosting by whisking together powdered sugar, cocoa powder and salt, if using, in a large bowl or mixer.
Add water, softened butter and vanilla extract, and beat together until smooth and creamy. Add more sugar for a thicker frosting and more water for a thinner frosting.
Remove cake from oven and let cool before frosting.
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