Preheat oven to 375 degrees F (190 degrees C) and spray two 9-inch fluted tube cake pans with cooking spray.

Spread half of the cajeta (or dulce de leche) into the bottom of each prepared cake pan.

In a blender, combine sweetened condensed milk, evaporated milk, 4 eggs, and vanilla extract.

Pulse several times to break up the eggs, then blend until thoroughly mixed (about 30 seconds).

Pour half of the milk mixture into each cake pan over the cajeta.

In a mixing bowl, combine the spice cake mix, water, vegetable oil, and 3 eggs.

Beat with an electric mixer on low speed for 30 seconds to combine.

Increase speed to medium and beat until the cake batter is smooth (about 2 more minutes).

Scrape half of the cake mix into each pan on top of the flan ingredients.

Bake in the preheated oven for about 45 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and immediately flip the pans over onto cake serving pans to remove the cakes.

Allow the cakes to cool completely before serving.