For the cake: In large bowl, beat cake mix, water, oil and 3 eggs with electric mixer on medium speed for two minutes, scraping bowl occasionally.
Pour batter over caramel topping in pan.
For the flan: In a blender, place flan ingredients, then cover. Blend on high speed about 20 seconds or until smooth.
Slowly pour mixture evenly over cake batter. (Flan mixture will mix with batter, but will separate while baking.)
Spray a piece of foil with cooking spray, placing the sprayed side down over the top of the pan, scrunching the edges to seal.
Place the bundt pan into a second pan that can accommodate the bundt pan and an inch of boiling water.
Bake 1 hour 20 minutes or until a toothpick inserted in center comes out clean.
Remove cake pan from water bath to cooling rack; remove foil. Cool at room temperature one hour. Refrigerate in pan, uncovered, two hours.
When ready to serve, remove the cake from the refrigerator. Place the serving plate upside down on the cake pan and invert both the plate and pan. You may need to jiggle the pan to get the cake out.
Remove pan.
Notes
Store any leftover cake in the refrigerator. Cajeta is a rich, creamy spread made from caramelized sugar and milk (traditionally goat's milk). It can be used in desserts or as a topping for a variety of sweet dishes. Sometimes referred to as "dolce de leche," you can find it in the Latin section of the supermarket, Latin corner stores, or online. If you aren't able to find it, you can substitute caramel sauce instead.