Preparation Preheat oven to 350°F. Coat cake pans with nonstick spray. Dust with flour, tapping out excess. Whisk cake mix, eggs, oil, and 1 cup milk in a large bowl until no lumps remain. Divide batter evenly among prepared pans; smooth tops. Bake cakes until a tester inserted into the centers comes out clean, 10-15 minutes. Meanwhile, mix coconut, sugar, sour cream, salt, and remaining 1 tablespoon milk in a medium bowl to combine; set frosting aside. Transfer pans to wire racks; let cool 5 minutes. Turn cakes out onto racks and spread frosting over tops of warm cakes, dividing evenly. Stack cakes. Let stand at room temperature, covered with a cake dome, or loosely tented with foil, at least 4 hours before serving. DO AHEAD: Cake can be made 2 days ahead. Store airtight at room temperature.
Preparation Preheat oven to 350°F. Coat cake pans with nonstick spray. Dust with flour, tapping out excess. Whisk cake mix, eggs, oil, and 1 cup milk in a large bowl until no lumps remain. Divide batter evenly among prepared pans; smooth tops. Bake cakes until a tester inserted into the centers comes out clean, 10-15 minutes. Meanwhile, mix coconut, sugar, sour cream, salt, and remaining 1 tablespoon milk in a medium bowl to combine; set frosting aside. Transfer pans to wire racks; let cool 5 minutes. Turn cakes out onto racks and spread frosting over tops of warm cakes, dividing evenly. Stack cakes. Let stand at room temperature, covered with a cake dome, or loosely tented with foil, at least 4 hours before serving. DO AHEAD: Cake can be made 2 days ahead. Store airtight at room temperature.