First, prepare the chocolate cake mix according to the package instructions.

Next, make the flan: In a blender, combine 1 can sweetened condensed milk, 1 can evaporated milk, 1/2 cup sugar, 1 tablespoon vanilla extract, and 4 eggs. Blend until smooth.

For the caramel, add 1 cup sugar and 3 tablespoons water to a skillet over medium heat. Stir and cook until the caramel turns a deep golden brown and is fully liquid and bubbling.

Pour the caramel into a cake pan, tilting the pan to coat the sides with caramel.

Grease the parts of the pan not covered by caramel with butter.

Pour in the chocolate cake batter.

Slowly pour the flan mixture from the blender into the pan, pouring it along one edge. The flan will sink below the cake batter.

Finally, place the cake pan inside a larger pan that fits it, and add water to the larger pan—just enough so the cake pan doesn't float, only a little water.