Preheat the oven to 350 degrees F. Grease a 12-cup nonstick bundt pan, taking care to get in all the nooks and crannies. Pour the caramel sauce into the bottom of the pan (the caramel sauce, if storebought and thick, may need a quick warmup in the microwave to be pourable).
In a medium bowl, combine the flour, cocoa, baking soda, and salt; set aside. In a microwave-safe bowl, combine the chocolate and butter and microwave at 50% power for 1-minute increments, stirring in between, until the mixture is melted and smooth, about 2-4 minutes. Whisk the buttermilk, sugar, eggs, and vanilla into the melted chocolate mixture until incorporated. Add the flour mixture and stir until combined. Pour the chocolate batter evenly over the caramel.
Add all the flan ingredients to a blender and process until smooth. Slowly and carefully pour the flan mixture over the the cake batter. Place the filled cake pan in a large roasting pan. Place the roasting pan in the oven and carefully pour warm water into the roasting pan until it reaches halfway up the sides of the bundt pan. Bake the cake until a toothpick comes out clean and the flan registers 180 degrees on an instant-read thermometer, about 75 to 90 minutes (mine took the longer amount of time). Remove the Bundt pan from the roasting pan (the easiest way for cleanup is to leave the roasting pan and water in the oven until it has cooled) and place it on a wire rack to cool completely. Refrigerate for 8 hours (or up to 12).
To remove the cake from the pan, fill a large bowl with hot water and place the bottom third of the Bundt pan in the water for 1 minute. Carefully invert the cake onto a flat plate or cake platter - I do this by putting the cake platter or plate over the Bundt pan opening and then, holding onto the edges of both platter and cake pan, I gently turn the pan upside down while flipping the cake platter. Slowly remove the pan letting the caramel drizzle over the top of the cake. Serve chilled.
Caramel Sauce: here is my favorite homemade recipe. Plan Ahead: the cake needs to chill for at least 8 hours before it can be turned out of the cake pan (perfect for making a day in advance!). Roasting Pan: also, that the cake needs to be baked in a water bath so you'll need a large roasting pan of sorts - I just used a disposable aluminum roasting pan. Bundt Pan: also, a 12-cup bundt pan is called for in the recipe; however, when I made it at my mom's house she only had a 9-10 cup bundt pan (which I didn't realize until I had already poured in the chocolate batter). I simply added the flan on top until it was about 1/2-inch from the top and discarded the rest of the flan mixture. It probably made for a slightly less thick flan layer but we absolutely loved it anyway.