Preheat oven to 350°F (175°C). Prepare cake mix according to package directions, replacing the water with cooled coffee for a rich mocha flavor.
Pour batter into a greased 9×13-inch baking dish and bake as directed on the box.
Once baked, remove from oven and let cool for 10 minutes. Using the handle of a wooden spoon, poke holes evenly across the cake.
In a bowl, whisk together sweetened condensed milk and chocolate syrup until smooth. Pour this mixture evenly over the cake, letting it soak into the holes.
Refrigerate cake for at least 2 hours to allow flavors to blend.
In a chilled bowl, whip heavy cream, powdered sugar, vanilla, and espresso powder until stiff peaks form. Spread evenly over cooled cake.
Dust with cocoa powder and garnish with chocolate shavings or chips before serving.
Notes
Make sure to use strong coffee for the best flavor.
This cake can be made a day in advance for a more developed taste.