This is from the Hershey's site:
http://www.hersheys.com/cookbook/

COCOA MAYONNAISE CAKE
Prep Time: 15 min.
Start to Finish: 3 Hrs.

Ingredients:

1-3/4 cups all-purpose flour
1 cup sugar
1/3 cup HERSHEY'S Cocoa or HERSHEY'S Dutch Processed Cocoa
1-1/2 teaspoons baking powder
1-1/2 teaspoons baking soda
1/8 teaspoon salt
1 cup mayonnaise
1 cup water
2 teaspoons vanilla extract
Instructions:
1. Heat oven to 350°F. Grease and flour 13x9x2-inch baking pan or 10-inch tube
pan.

2. Stir together flour, sugar, cocoa, baking powder, baking soda and salt in
large
bowl. Add mayonnaise, water and vanilla; beat on medium speed of electric
mixer 3 minutes. Pour batter into prepared pan.

3. Bake 40 minutes for rectangular pan or 45 to 50 minutes for tube pan or until
wooden pick inserted in center comes out clean. Cool 10 minutes; remove from
pan. Cool completely; frost as desired. 12 to 15 servings.