You will need a 12-cup bundt pan and a roasting pan large enough to put the bundt pan inside of and have plenty of room for water. You are creating a Bain Marie or water bath to cook the flan layer.
Set one oven rack in middle of the oven. Preheat oven to 350 degrees F. Coat the bundt pan with butter and drizzle the bottom of the pan with cajeta.
Boil about 2 quarts of water for the Bain Marie.
For the Chocolate Cake Layer: With an electric stand mixer, cream the butter and sugar together until light and fluffy, 3-5 minutes. Beat in the egg. Sift all dry ingredients together in a separate bowl. Then mix the dry ingredients into the wet mixture, 1/3 at a time, alternating with buttermilk until all ingredients are incorporated.
For the Flan Layer: Place all the flan ingredients together in a blender and blend on high until smooth, about 30 seconds.
Set the bundt pan inside the roasting pan. Scoop cake batter into the bundt pan on top of the cajeta, smooth it out. Then gently pour the flan mixture over the back of a large spoon into the bundt pan so that it lays on top of the cake batter.
Lightly butter a piece of foil and cover the bundt pan tightly, butter side down against the cake. Pour boiling water in the roasting pan about two inches high. Gently slide the pan onto the middle oven rack. Bake for 1 hour without opening the oven door. (You need to keep all of the steam inside the oven to do its job.)
Remove the foil and test the cake. It should be firm to the touch, and an inserted toothpick should come out clean. (If not, place back in the oven for another 10-20 minutes.) Remove the cake from the water bath and cool completely to room temperature. When cool, shake the pan from side to side to loosen, place a large platter inverted over the cake, hold tightly and quickly flip over. With a rubber spatula scrape the remaining cajeta onto the cake.
Serve at room temperature, or chill and serve cold. Both are !Muy Delicioso! Garnish with more cajeta if desired.
Cajeta is easy to find in most grocery stores. Check the Mexican food section or Mexican market. You can also find it online. If you can't find it you may substitute Dulce de Leche or another type of caramel sauce.