Remove the paper (on the outside of the can) from 1 can of sweetened condensed milk.
Make the dulce de leche: Add the can to a small saucepan and cover with water. Bring to a boil over medium-high heat and let boil for 3 hours. Continue adding water to keep the can covered while boiling. Once it has boiled for 3 hours, remove from heat and let cool. (See notes for slow cooker method.)
Preheat the oven to 350°F. Spray a 9x13” baking dish with baking spray and set aside.
Add eggs and sugar to the bowl of a stand mixer fitted with the whisk attachment. Mix together on medium speed for 5 minutes, until fluffy. Add in the oil, sour cream and vanilla and mix till well combined.
Add in flour, cake flour, baking powder, baking soda and salt and mix until just incorporated. (Do not over mix.) Transfer the batter into the prepared baking pan and smooth out the top with a spatula.
Place the cake into the preheated oven on the center rack and bake for 35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Once baked, remove from the oven.
Allow the cake to cool down for 10 minutes, then poke holes into the top of the cake using a fork, large toothpick, or chopstick.
Pour the uncooked can of sweetened condensed milk over top and spread it out evenly over the top of the cake. Let it soak in. Then pour the boiled can of sweetened condensed milk (Dulce de Leche) on top and spread it out evenly.
Cover the cake and place into the fridge to chill for 8 hours before serving.
Slow Cooker Dulce de Leche: Place can of sweetened condensed milk in the crockpot. Cover with water until water is 2 inches over the top of the can. Place the lid on the crockpot and cook for 9 hours on low. Remove can from crockpot and let cool.
Boiling the Sweetened Condensed Milk: If you want to skip boiling the sweetened condensed milk, you can simply buy a can of dulce de leche to use instead, or use my homemade dulce de leche over the top. (If you do that, I recommend cooking it a little longer till it’s extra thick.)
Mixing and Overmixing: Once the dry ingredients are added to the recipe, make sure you only mix until just combined. Overmixing can cause the gluten to overdevelop, resulting in a crumbly cake.
Adding the Condensed Milk and Dulce de Leche: Don’t let the cake cool completely before poking the holes and adding the sweetened condensed milk and dulce de leche. Adding these ingredients while the cake is still warm helps them soak in better, resulting in a rich, moist cake.
Cinnamon Option: Adding 2 teaspoons of cinnamon to the cake is also really delicious! It gives the cake a beautiful warmth.
Extra Toppings: I didn’t feel the need to top the cake with anything more than the thick dulce de leche, but if you want, you could spread some whipped cream or Cool Whip on top like you would with a tres leches cake.