Preheat oven to 350 degrees. Spray a 9"x13" baking dish with cooking spray.
Beat chocolate cake mix, egg and water in a large mixing bowl, on medium speed for about 1 minute. Gently fold in cherry pie filling.
Pour batter into prepared baking dish and bake for about 25 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before frosting.
For the frosting, beat together butter and cream cheese until smooth, about a minute. Slowly add sifted powdered sugar, salt and cocoa, then beat on high for 1-2 minutes, until fluffy. Stir in vanilla.
Spread immediately on the cooled cake. Store leftovers, covered, in the refrigerator.