Preheat oven to 350. Prepare a standard 12-cup Bundt pan (spray and lightly flour).
In a large mixing bowl, blend together cake mix, buttermilk, applesauce, egg whites and flour, stirring with rubber spatula to ensure all ingredients are mixed together well.
Pur the batter into the prepared pan and back until cake is golden brown and springs back when lightly touched (35-40 minutes). Remove pan from oven and let cool for 20 minutes. Then invert onto a wire rack to remove cake. Place lightly cooked cake on serving plate, and poke holes in the top of the cake with a toothpick. Brush cake generously with sweeted condensed milk. Lightly cover and refrigerate at least 4 hours.