Preheat your oven to 350F. Grease an 8x8" glass baking pan and set it aside (if you have an aluminum pan, line it with parchment paper to avoid a metallic taste).
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda and salt. Create a well in the center and add the water, oil, vinegar and vanilla. Stir it all together until just combined and the batter is smooth. Pour the batter into the prepared baking pan. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Set the pan on a wire rack to cool it before frosting.
For the icing: whisk together all the icing ingredients in a medium bowl until smooth. It should be thick and fudgy, but still spreadable. If it's too thick, add more water 1 tsp at a time until you get the right consistency. Once the cake is completely cooled, gently spread the icing over the top, then slice the cake into squares and serve.
