Prepare the mold. Turn on the oven to 350 degrees and position the rack in the middle. Generously butter the inside of a 10-cup Bundt pan. Microwave the cajeta for 20 to 25 seconds just to soften it (not heat it), then drizzle it evenly over the bottom half of the pan, tilting the pan to coat everything evenly. (Don’t worry that most of it will eventually collect in the very bottom.) Set large pan of water over medium-low heat. Set out a deep pan that's a little larger than your Bundt pan (a roasting pan works well) and deeper than 2 inches; this will serve as a water bath during baking.
Prepare the cake batter. In a medium bowl, whisk together the flour, sugar, cocoa, salt, baking powder and soda. In another bowl, whisk together the milk (or milk and coffee), egg and oil. Whisk the liquid ingredients into the dry ingredients until the two are well blended. Spoon the mixture into the prepared pan, then smooth it on top.
Prepare the flan. In a blender, combine the two milks (scrape everything out of the sweetened condensed milk can), vanilla and the eggs. Blend just until the mixture is homogenous. Rap the blender jar gently on the countertop 6 or 8 times to expel air bubbles. Slowly and gently, pour the flan mixture over the cake batter. (I find it easiest to pour the mixture into a small ladle, letting it run over onto the batter; others prefer pouring it over a spoon.) Oil or butter a piece of foil and use it to cover (greased-side down) the pan, tamping it down all around the top edge to secure it.
Bake. Gently move the cake pan to the larger pan and pour hot water around the cake pan to a depth of 2 inches (no less). Carefully slide the whole assemblage into the oven. (Some people find it easiest to pull out the oven rack, set the cake pan in the large pan on it, pour in the water then slide the rack back in.) Bake about about 1 hour and 15 minutes, until the surface of the cake is firm to the touch and a toothpick inserted in the middle comes out relatively clean. Remove from the water bath and cool to room temperature, about 1 hour. Run a thin-bladed knife around the upper edges and center of the bundt pan to loosen the cake from the mold. It’s safest to unmold the chocoflan after it has chilled long enough to firm it, so I recommend refrigerating it for several hours or overnight.
Unmold and serve. Just to be sure nothing sticks, run your thin-bladed knife around the upper part of the bundt pan (don’t forget to do the center) for a second time. With the pan firmly on the countertop, jiggle it back and forth a few times until you see a slight movement in the cake. Invert a serving plate over the mold, then reverse the two. If the cake doesn’t drop immediately, give the mold a few raps until you hear it fall. Remove the mold. Scrape any cajeta remaining in mold over the chocoflan and you’re ready to cut and serve slices to guests who likely won’t know what a seemingly impossible thing happened while flan and batter were in the oven.