Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour.
In large bowl, place cake mix. In 1-cup glass measuring cup, stir 1 cup warm water and 1 tablespoon espresso granules until granules are dissolved. Add espresso mixture, butter and eggs to cake mix. Beat with electric mixer on low speed 30 seconds; scrape bowl. Beat on medium speed 2 minutes longer. Pour batter into pan.
Bake as directed on box for 13x9-inch pan. Cool in pan on cooling rack 15 minutes.
Meanwhile, spoon dulce de leche into medium microwavable bowl. In small bowl, mix 1/2 cup hot water, 3 tablespoons espresso granules and the rum; stir into dulce de leche until smooth. Microwave uncovered on High 2 to 3 minutes, stirring after about 1 minute with whisk, until pourable. Set aside while cake cools.
Poke cooled cake every 1/2 inch with handle end of wooden spoon. Pour dulce de leche mixture evenly over cake; spread mixture over top of cake with metal spatula to fill holes. Run knife around sides of pan to loosen cake. Cover; refrigerate 2 hours.
In medium bowl, beat whipping cream and powdered sugar on high speed until stiff. Spread whipped cream evenly over chilled cake. Sprinkle with chopped chocolate. Store covered in refrigerator.