Preheat oven to 350 degrees F. Liberally grease a 12-cup Bundt pan with cooking spray. Pour the caramel sauce in the bottom of the Bundt pan evenly. Set aside briefly.
In the bowl of a stand mixer, whisk together the flour, cocoa powder, baking soda, and salt until blended. Meanwhile, in a microwave-safe medium-size bowl, heat the chopped chocolate bar and butter together on HIGH power for 30 seconds. Stir, then heat again for another 30 seconds or until melted and smooth.
Whisk the sugar into the melted chocolate mixture, and pour the melted chocolate mixture into the stand mixer bowl with the dry ingredients. Beat slowly, then stream in the whole milk. Beat until combined, followed by adding the eggs, one at a time, and the vanilla. Pour the chocolate cake batter into the prepared pan in an even layer.
In a large blender, add the flan ingredients and process for 1 minute or until thin and combined. Pour the flan mixture evenly over the chocolate cake batter in the pan; the pan will be very full. Carefully transfer the Bundt pan to a larger roasting pan and nestle it inside. Pour hot water into the roasting pan until it hits about halfway up the Bundt pan.
Bake for approx. 75-90 minutes or until a toothpick or cake tester inserted into the cake and through the flan comes out clean. Cool completely, then refrigerate for at LEAST 8 hours, preferably overnight. This is MANDATORY.
Just before serving, put the Bundt pan back into the roasting pan and add about 1-2 inches of hot tap water. Let sit in the water for about 1-2 minutes. Take a platter or cake stand and flip it upside down on top of the Bundt pan. Working quickly and carefully, invert the Bundt pan onto the cake stand/platter (whichever you're using) and let set for a second so all the caramel drips onto the flan cake. Remove the Bundt pan.
Serve while cold and store leftovers covered in the fridge. Spoon any extra caramel in the Bundt pan over the top of the flan.