Preheat the oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking pan.

Combine cake mix, undrained fruit cocktail, 1 cup coconut, and eggs in a large bowl. Beat with an electric mixer on medium speed for 2 minutes. Pour batter into the prepared pan, then sprinkle brown sugar and walnuts evenly over the surface.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes.

When the cake is almost done, combine butter, white sugar, and evaporated milk in a saucepan; bring to a boil. Boil for 1 minute, then stir in remaining 1/2 cup coconut.

Spoon hot coconut mixture over warm cake.