Prepare the cake mix as directed on the box in a 9x13 dish. Let cool.
Take a fork and pierce the cake many times. This will allow the "leches" to seep in nicely.
Mix the evaporated milk and condensed milk and pour over the cake as evenly as possible. Chill in the fridge for a couple of hours.
Whipped topping: Use an electric mixer to beat the heavy whipping cream with sugar and vanilla. Beat until stiff peaks form. Spread on top of chilled cake and return to the fridge until it's time to serve.