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.
Don't want chocolate frosting? Omit the cocoa powder and use extra powdered sugar if needed to reach the desired texture.