Preheat your oven to 350°F (175°C). Generously grease the Bundt pan with butter or non-stick spray to ensure easy release.
Spread the dulce de leche or caramel sauce evenly across the bottom of the prepared Bundt pan.
In a large bowl, combine the evaporated milk, sweetened condensed milk, 4 eggs, and vanilla. Whisk until completely smooth and free of lumps. Carefully pour this mixture over the caramel layer in the pan.
Prepare the chocolate cake batter according to the package instructions using the required oil, water, and eggs.
Slowly pour the chocolate cake batter directly over the custard mixture. Do not worry if they seem to mix slightly; they will separate during the baking process.
Cover the Bundt pan tightly with aluminum foil to prevent the top from drying out.
Place the Bundt pan into the large roasting pan and fill the roasting pan with hot water until it reaches halfway up the sides of the Bundt pan.
Bake for 60 to 70 minutes. The cake is done when a wooden skewer inserted into the chocolate portion comes out clean.
Remove from the water bath and let the pan cool on a wire rack for at least 60 minutes.
Refrigerate the cake in the pan for at least 4 hours, or preferably overnight, before carefully inverting it onto a serving platter.