Preheat your oven to 350°F (175°C) and grease a 9×13 baking pan with nonstick spray. This helps ensure the cake comes out easily after baking.
In a mixing bowl, combine the white cake mix, water, vegetable oil, and egg whites. Use a hand mixer on medium speed for about 2 minutes until the batter is smooth and well combined. Look for a creamy consistency without lumps.
Pour the batter into your prepared pan, ensuring it's evenly distributed. Bake for 29 to 33 minutes, or until a toothpick inserted into the center comes out clean. The top should be lightly golden.
Once baked, remove the cake from the oven and let it cool for about 10 minutes. While it’s still warm, take the handle of a wooden spoon and poke holes into the cake about 1 to 2 inches apart, going halfway down.
In a separate bowl, whisk together the cold coffee, instant coffee, and sweetened condensed milk until combined. Slowly pour this mixture over the warm cake, allowing it to seep into the holes you've created. Refrigerate for at least 1 hour to ensure it's completely cool and soaked through.
In another mixing bowl, beat the mascarpone cheese and whipped topping together until smooth and creamy. This step is crucial for achieving that luxurious tiramisu texture.
Spread the creamy mixture evenly over the chilled cake. Look for a smooth, even layer that covers all the holes. This is where the magic happens!
For an optional garnish, use a microplane or fine grater to shave unsweetened chocolate over the top of the cake, adding a touch of elegance.
Slice and serve immediately, or cover and refrigerate until you're ready to enjoy. This cake gets better as it sits, so don’t hesitate to make it ahead of time!