Prepare the cake mix according to package directions and bake in a 9x13 pan. Remove from oven and cool 5 minutes.
Combine condensed milk, milk, cream, and rum.
Using a skewer or a fork, poke holes in the top of the cake. Slowly pour milk mixture evenly over the top of the cake. Let cook a few more minutes and then refrigerate several hours or overnight
To make topping: Whip heavy cream with sugar and vanilla until very stiff. Spread over the top of the cooled cake. Refrigerate until ready to serve....it only gets better as it sits.