Melt the sugar to make caramel and pour it into a cake pan.

For the cake layer, melt the semisweet chocolate with the butter using a double boiler and set aside.

In a separate bowl, beat the eggs with the sugar until thick and pale. Once the mixture has increased in volume, slowly add the melted chocolate and butter (cooled to room temperature) in a thin stream while mixing. Stop beating and gently fold in the sifted dry ingredients, being careful not to deflate the batter. Pour this mixture over the caramel in the pan.

For the flan, blend the sweetened condensed milk, evaporated milk, eggs, vanilla, and cornstarch until smooth. Carefully pour this mixture over the cake batter. Bake in a water bath with hot water for about 1 hour and 30 minutes, or until a toothpick inserted in the center comes out clean, at 400°F (200°C).