How to Make It
Preheat oven to 350°F. Spray a 15- x 10-inch jelly-roll pan liberally with baking spray. Combine, cake mix, water, oil, and eggs in the bowl of a heavy-duty electric stand mixer, and mix according to package directions. Stir in carrots, pineapple, and 1 cup of the walnuts. Pour batter into prepared jelly-roll pan. Bake in preheated oven until a wooden pick inserted in center comes out with moist crumbs, about 25 minutes.
Transfer pan to a wire rack; cool completely, about 30 minutes.
Beat cream cheese, butter, and vanilla with a heavy-duty electric stand mixer fitted with the whisk attachment on medium speed until fluffy and well combined, 4 to 5 minutes. Add powdered sugar, 1 cup at a time, beating until blended after each addition and stopping to scrape down sides of bowl as needed.
Spoon cream cheese mixture, in dollops, over surface of cooled cake. Using an offset spatula, spread to coat surface of cake evenly. Top evenly with remaining 1 cup walnuts. Sprinkle evenly with coconut.