Step 1: Preheat your oven to 350°F (175°C). Generously grease a 10-inch Bundt pan and pour the caramel sauce into the bottom, coating it evenly.
Step 2: In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
Step 3: In a large bowl, cream the butter and sugar until light and fluffy. Beat in the egg and vanilla. Alternately add the dry ingredients and buttermilk, beginning and ending with the dry ingredients.
Step 4: Spread the chocolate cake batter evenly over the caramel in the Bundt pan.
Step 5: In a blender, combine the evaporated milk, condensed milk, 4 eggs, and vanilla. Blend until smooth.
Step 6: Slowly pour the flan mixture over the back of a spoon onto the cake batter. Cover the pan tightly with aluminum foil.
Step 7: Place the Bundt pan in a large roasting pan. Fill the roasting pan with hot water to reach halfway up the sides of the Bundt pan.
Step 8: Bake for 60-70 minutes, or until a toothpick inserted into the cake comes out clean.
Step 9: Remove from the water bath and let cool to room temperature for 1 hour. Refrigerate for at least 4 hours, or overnight.
Step 10: To unmold, run a knife around the edges, place a serving plate over the pan, and carefully invert. Let the caramel drip down the sides before serving.