Preheat oven to 350°F (175°C). Grease a Bundt pan.
Prepare the chocolate cake mix according to the package instructions. Pour the batter into the prepared Bundt pan.
In a blender, combine sweetened condensed milk, evaporated milk, eggs, and vanilla extract. Blend until smooth.
Slowly pour the flan mixture over the cake batter in the Bundt pan.
Cover the Bundt pan with aluminum foil and place it in a larger baking dish. Fill the larger dish with about 1 inch of hot water to create a water bath.
Bake for about 60 minutes, or until a toothpick inserted into the cake comes out clean.
Remove the Bundt pan from the water bath and let it cool to room temperature. Refrigerate for at least 4 hours or overnight.
To serve, invert the cake onto a serving plate and drizzle with caramel sauce.