Preheat the oven to 350°F (175°C). Combine the cake mix, eggs, water, and oil in a large bowl, stirring until the batter is smooth and fully blended. Pour the batter into a greased 9×13-inch pan. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool for approximately 10 minutes before moving on to the next step.
While the cake is still warm, poke holes across the surface using the handle of a wooden spoon or a skewer. Gradually drizzle the sweetened condensed milk over the holes, allowing it to absorb into the cake. If using, drizzle caramel sauce evenly over the top.
In a medium saucepan set over medium heat, combine the evaporated milk, sugar, and butter. Heat the mixture to a gentle boil, stirring continuously. Lower the heat and let it simmer, stirring occasionally, until it thickens—about 10 to 12 minutes. Stir in coconut, pecans, and vanilla extract.
Spread the warm coconut-pecan frosting evenly over the soaked cake. Use a spatula to spread evenly. Allow the cake to cool to room temperature before refrigerating it for at least 2 hours. Optional: Spread whipped topping over the frosting for extra creaminess.
