Preheat oven to 350 degrees for a metal or glass pan or 325 degrees for dark or coated pans.
In a large mixing bowl, combine cake mix, water, oil and eggs until combined and smooth. Pour batter into a well greased 9x13 inch baking pan and cook for 26-30 minutes, or until an inserted toothpick comes out clean. Let cool completely.
After completely cooled, remove the cake from the pan and cut in half horizontally. Replace the bottom half of the cake into the pan. Cut ice cream into even slices and place over the bottom cake layer. Put the top half of the cake over the layer of ice cream. Cover and store in the freezer until ready to eat. Cut into slices and top with hot fudge, whipped cream, nuts and a cherry.
In a large sauce pan, combine butter, cocoa, sugar and evaporated milk over medium heat.
Bring to a boil and let boil for 7 minutes, stirring occasionally. Remove pan from heat and whisk in vanilla. Serve immediately.
Store extra hot fudge in the refrigerator and microwave to reheat as needed.