Preheat oven to 350 degrees F (176 C).
Spray a bundt pan with some baking spray, then coat the bottom of the bundt pan with the caramel sauce.
For the cake:
In a mixing bowl fitted with a paddle attachment, mix butter and sugar together until light and fluffy. Next add the egg. In a separate bowl, sift all the dry ingredients together, then alternately add dry ingredients and buttermilk to cake batter until fully incorporated.
Pour cake batter into bundt pan directly over the caramel sauce.
For the flan:
Combine all ingredients in a blender. Blend for 30 seconds.
Pour flan base over cake batter in the bundt pan.
Cover bundt pan with foil, then place in a roasting pan with 1 inch of hot water inside. If you do not have a roasting pan, you can just place another pan with water inside the oven along with the bundt pan to create steam.
Bake for 50 minutes to 1 hour, or until a toothpick comes out clean.
When fully baked, remove from water bath and let it cool in the bundt pan for 1 hour.To invert, place a serving platter over the bundt pan and flip over. It should jiggle a little then come out clean.
Feel free to garnish with some more caramel sauce!