Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan with non-stick spray or butter. Prepare the yellow cake mix according to the package instructions. Once mixed, pour the batter into the prepared baking pan and smooth the top with a spatula.
Bake the cake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 10-15 minutes, then transfer it to a wire rack to cool completely.
In a medium bowl, combine 1 can of evaporated milk, the sour cream, and 2 teaspoons of water. In a separate bowl, take half of the dulce de leche (about 6.7 oz.) and mix it with the milk mixture until well combined.
Once the cake has cooled completely, use a fork or skewer to poke holes all over the surface. This will allow the milk mixture to soak in. Slowly pour the milk mixture evenly over the cake, ensuring it seeps into the holes. Allow the cake to sit for at least 30 minutes to absorb the liquid.
In a large bowl, take the thawed COOL WHIP Whipped Topping and fold in the remaining dulce de leche until well blended. This will create a creamy, caramel-flavored topping for your cake.
Spread the dulce de leche whipped topping evenly over the soaked cake using a spatula. You can also create swirls or peaks for a beautiful presentation.
Refrigerate the cake for at least 2 hours, or overnight for best results. This allows the flavors to meld beautifully. When ready to serve, slice into squares and enjoy your rich and creamy dessert!