Preheat oven to 350°F. Grease and flour a 9-inch round cake pan.

In a large mixing bowl, whisk together flour, sugar, cocoa powder, and baking soda.

Add water, mayonnaise, and vanilla extract to the dry ingredients. Mix until smooth.

Pour batter into prepared cake pan and bake for 30 minutes, or until a toothpick inserted into the center comes out clean.

Allow cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.