Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking dish.

Sift together flour, cocoa powder, baking soda, and salt in a large bowl. Add sugar and mix until well combined.

Pour in water, oil, and vanilla; mix thoroughly until batter is smooth. Pour batter into the prepared baking dish.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour.