Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, combine the yellow cake mix, eggs, vegetable oil, and water. Beat with an electric mixer on medium speed for 2 minutes.
Pour batter into the prepared baking pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
While the cake is still warm, use the handle of a wooden spoon or a fork to poke holes all over the top of the cake.
In a separate bowl, whisk together the sweetened condensed milk and instant vanilla pudding mix. Gradually whisk in the cold milk until well combined and slightly thickened.
Pour the pudding mixture evenly over the warm cake, allowing it to seep into the holes.
Cover the cake and refrigerate for at least 2 hours, or until thoroughly chilled.
Spread the thawed whipped topping evenly over the chilled cake. Sprinkle with shredded coconut, if desired.
Slice and serve cold.
Notes
This poke cake is best served cold. You can also add fresh fruit on top for an extra burst of flavor.