Preheat oven to 350°F. Spray a 9x13-inch baking dish with nonstick spray and set aside.
Mix the cake mix, espresso (if using), water, oil, and eggs together in a medium bowl.
15.25 ounces chocolate cake mix, 1 teaspoon instant espresso, 1¼ cups water, ½ cup vegetable oil, 3 large eggs
Transfer the cake batter into the prepared baking dish and place into the preheated oven on the middle rack. Bake for 48 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Once baked, remove the cake from the oven and place on a wire cooling rack.
While the cake is warm, poke holes into the cake using the handle of a wooden spoon or a clean dowel rod. Set aside while you make the milk mixture.
In a medium-sized bowl, whisk the hot milk, sweetened condensed milk, evaporated milk and cocoa together.
1 cup milk, 14 ounces sweetened condensed milk, 12 ounces evaporated milk, ⅓ cup unsweetened cocoa powder
Once well combined, pour over the warm cake, then let the cake finish cooling.
Once cooled, cover and place in the refrigerator to chill for at least 4 hours, but preferably overnight.
In the bowl of a stand mixer fitted with the whisk attachment, combine the heavy cream, powdered sugar, cocoa, and vanilla. Beat for 4-5 minutes, until stiff.
2½ cups heavy cream, ½ cup powdered sugar, 4½ tablespoons unsweetened cocoa powder, 1 teaspoon pure vanilla extract
Spoon the topping onto the chilled, milk soaked cake. Drizzle lines of chocolate sauce overtop and pull with a toothpick.
Chocolate sauce