Mix together the cake mix, ingredients called for on the box, plus ⅓ cup sweetened condensed milk. Pour the batter into a well greased bundt cake pan. Bake according to box instructions for a bundt cake.
Once baked, remove from the oven and let cool for 15-20 minutes. Then, carefully using a spatula, gently pull the edges of the cake away from the walls of the pan to loosen it, but keep the cake in the pan.
Using the wide end of a chopstick or the end of a small wooden spoon, poke holes all over the bottom of the cake. Pour the remaining sweetened condensed milk over the cake and allow it to seep into the holes, about 10-15 minutes.
Cover and transfer to the refrigerator to finish cooling and allow the condensed milk to absorb, about 2 hours.
Carefully flip the cake from the pan onto a serving plate. Enjoy!
OPTIONAL: You can top the finished cake chocolate ganache or top/serve with whipped topping!