Make the flan: Add all the flan ingredients except the sugar to a blender. Blend until completely smooth, then set aside.

Make the cake: In a bowl, combine all the cake ingredients. Use a mixer to beat until the batter is smooth and no lumps remain, about 3 minutes. Set aside.

If you want to use caramel: Add the sugar to a nonstick skillet and heat over low. Stir constantly until the sugar melts and turns into a golden caramel. Be very careful, as it will be extremely hot.

Immediately pour the caramel into the cake pan, tilting to coat the bottom evenly. Lightly grease the sides of the pan with a little butter to prevent sticking. (You can skip the caramel step and simply spread cajeta over the bottom of the pan instead.)

Now, pour the mixtures into the pan—here’s the “impossible” part! First, pour the chocolate cake batter over the caramel. Then, immediately pour the flan mixture on top of the cake batter. It may look like everything is mixed together, but don’t worry—it will separate as it bakes.

Cover the pan very well with aluminum foil. Bake in a water bath (bain-marie) at medium heat, either in the oven or on the stovetop. It usually takes about 1 1/2 to 2 hours, depending on your oven or stove. To check if it’s done, insert a thin knife—if it comes out clean, it’s ready. If not, bake a little longer.