Preheat your oven to a toasty 350 degrees F (175 degrees C), then generously grease and flour a 15x10-inch jelly roll pan.
In a large bowl, mix cake mix, water, eggs, and oil with an electric mixer on low until moistened. Increase speed to medium and beat for 2 minutes. Pour batter into the prepared pan.
Bake in the preheated oven for 24 to 29 minutes until a toothpick inserted into the center comes out clean. Allow to cool completely for at least 45 minutes.
In a bowl, mix sugar, shortening, evaporated milk, butter, vanilla extract, and salt. Beat with a hand mixer until fluffy and light for 10 to 15 minutes. Pipe the cream frosting in lines on the cooled cake.
Put the chocolate frosting in a microwave-safe bowl and heat it in the microwave for 1 minute, pausing to check after 30 seconds. Use a spoon to drizzle the melted frosting over the cream. Chill in the refrigerator until it sets, approximately 1 hour.