Preheat and prep: Heat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment for easy lifting.
Make the cake: Prepare the chocolate cake mix according to the box, or use your homemade batter. Spread it evenly in the pan.
Bake: Bake for the time listed on the box or recipe, usually 25–35 minutes, until a toothpick comes out clean or with a few moist crumbs.
Cool slightly: Let the cake rest for 10–15 minutes.
It should still be warm, not hot, when you poke it.
Poke the holes: Using the handle of a wooden spoon or a thick straw, poke holes across the cake about 1 inch apart. Go almost to the bottom but don’t break through the base.
Mix the soak: In a bowl, whisk the sweetened condensed milk, heavy cream, chocolate syrup (or melted chips), vanilla, and a pinch of salt until smooth and pourable.
Pour and spread: Slowly pour the mixture over the warm cake, aiming for the holes first, then the top. Use a spatula to gently nudge it into any dry spots.
Chill: Cover the pan and refrigerate for at least 2 hours, or overnight for best texture.
The cake will absorb the liquid and set up.
Make the topping: Beat the cold heavy cream, powdered sugar, cocoa, and vanilla to medium-stiff peaks. It should be fluffy and spreadable.
Finish and serve: Spread the whipped topping over the chilled cake. Add chocolate shavings or mini chips if you like.
Slice into squares and serve cold.