1. Make the caramel:

In a small saucepan, melt the sugar over medium heat until it turns golden amber.
Quickly pour the caramel into a bundt or round cake pan (20–22 cm diameter).
Tilt the pan to coat the bottom evenly. Let it cool and harden.

2. Prepare the flan:

In a blender, mix the sweetened condensed milk, evaporated milk, eggs, and vanilla until smooth.
Pour this mixture over the hardened caramel in the pan.

3. Prepare the chocolate cake:

Make the chocolate cake batter according to the box or your recipe.
Carefully pour the batter over the flan mixture. (They will seem to mix, but will separate during baking!)

4. Bake in a water bath (bain-marie):

Preheat oven to 180°C (350°F).
Place the cake pan inside a larger baking dish and pour hot water into the larger dish, about halfway up the sides of the cake pan.
Bake for about 1 hour, or until a toothpick inserted into the chocolate layer comes out clean.

5. Cool and unmold:

Remove from oven and let cool to room temperature.
Chill in the fridge for at least 4 hours or overnight.
To unmold: run a knife around the edges, place a serving plate over the pan, and invert quickly.