Preheat oven to 350°F (175°C).
Prepare the chocolate cake batter according to package directions.
Pour into a greased 9x13-inch pan and bake for 28–32 minutes, or until a toothpick comes out clean.
Remove from oven and let cake rest for 15 minutes. It should be warm but not hot.
Using the handle of a wooden spoon or skewer, poke holes about 1 inch apart across the surface.
Make sure holes are deep so the filling seeps in.
In a bowl, whisk together the chocolate milk and whipped topping until smooth and creamy.
Slowly pour the mixture over the cake, letting it sink into the holes.
Spread gently to cover evenly. Let sit for 10 minutes.
Melt semisweet chocolate chips until smooth.
Drizzle or spread evenly over the cake for a glossy finish.
Cover and refrigerate for at least 2 hours, or overnight for best results.
Slice into squares and serve chilled.
