Step 1

Preheat oven to 350°F. Spray a 9x9-inch baking dish with nonstick cooking spray.

Step 2

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

Step 3

Add the water, mayonnaise, and vanilla. Mix until evenly combined.

Step 4

Pour the batter into the prepared baking pan. Bake for 25-30 minutes, or until a toothpick comes out mostly clean.

Step 5

Let the cake cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.