How to Make It
Preheat oven to 350°F. Spray a 9-inch cake pan with baking spray with flour, and line the bottom with parchment paper.
Whisk together flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl until incorporated. Add water, oil, and vanilla, and whisk until well combined. Pour cake batter into prepared cake pan, and bake in preheated oven until a wooden toothpick inserted comes out clean, 30 to 32 minutes.
Let cool on a wire rack until completely cooled, about 30 minutes. Invert onto a serving platter. If desired, top with berries and dust with powdered sugar.
If using Frosting: Whisk together powdered sugar and cocoa powder until well combined. Whisk in water, vanilla, and salt, and mix until smooth. Spread icing over cooled cake.