Prepare the Caramel:
In a pan over medium heat, melt the sugar until it turns golden. Be careful not to let it burn! Pour the hot caramel into a mold and let it cool.
Mix the Cake Batter:
Prepare the cake by following the instructions on the boxed mix, combining eggs, oil, and water until smooth. Pour this mixture over the cooled caramel in the mold.
Make the Flan Mixture:
In a blender, combine the condensed milk, evaporated milk, eggs, and vanilla essence until homogeneous. This will be poured over the chocolate cake batter.
Bake in a Water Bath:
Cover the mold with foil. Place it in a larger tray filled with hot water, halfway up the mold's sides. Bake at 175°C (350°F) for about 1 hour and 15 minutes, or until a toothpick inserted in the flan comes out clean.
Cool and Unmold:
Allow the chocoflan to cool completely, then refrigerate for at least 4 hours. To serve, run a knife around the edges and gently flip it onto a large plate—voilà!