Preheat the oven to 350 degrees F. Grease a 9 x 13-inch baking pan.
In a large bowl add the cake mix, sour cream, evaporated milk, eggs, and butter. Mix with an electric mixer on medium speed for 2 minutes. The batter will be thick.
Spread the cake in the prepared baking pan. Place in the preheated oven and bake for 20 to 25 minutes or until a wooden pick comes out clean after being inserted in the cake.
While the cake is baking prepare the hot fudge. In a microwave-safe bowl or measuring cup combine the sugar, cocoa, cornstarch, and salt mix. After the ingredients are mixed together stir in the water.
Microwave for 3 minutes stirring after 1 1/2 minutes. Add the butter and mix it in. Continue to microwave for another 1 1/2 minutes stirring after 45 seconds. At this time the hot fudge should be thick, if it is too runny microwave a little longer. Stir in the vanilla.
Once the cake is baked remove from the oven and with a wooden spoon poke holes over the whole cake. Spread the hot fudge over the top of the cake making sure that the hot fudge goes into the holes.
Cool to room temperature. Once cooled set in the refrigerator for 1 to 2 hours or until the cake is well chilled.
When the cake is cooled make the whipped cream. Add the whipping cream to a large bowl with an electric mixer beat until there are soft peaks. Add the cocoa and powdered sugar and continue to beat until stiff peaks form, Do not overbeat.
Spread the whipped cream over top of the hot fudge. Refrigerate until ready to serve