Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking dish.

Mix together cake mix, oil, beaten eggs, pudding mix, and Mountain Dew in a large bowl until well combined. Pour into the prepared baking dish.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Allow cake to cool completely on a wire rack.

Combine pineapple with juice, sugar, and cornstarch in a saucepan. Bring to a boil and cook, stirring constantly, until thick. Remove from heat. Stir in coconut and butter. Spread evenly over cooled cake.