Bake the cake. Prepare the cake mix according to box instructions (or your favorite recipe). Pour into a greased 9×13-inch pan. Bake as directed. Let it cool slightly, about 10 minutes—don’t burn your fingers testing it!
Poke the cake. Grab a wooden spoon handle or a skewer and poke holes all over the warm cake. We’re talking evenly spaced, not a random stabbing spree—though honestly, a few extra holes won’t hurt.
Make the pudding. In a medium bowl, whisk together pudding mix, cold milk, and vanilla. Whisk until smooth and thickened—should take 2–3 minutes.
Pour pudding over cake. Gently pour the pudding over the poked cake, making sure it seeps into all those little pockets. Smooth the top with a spatula so every bite is chocolatey bliss.
Chill. Refrigerate the cake for at least 2 hours (or overnight if you have the patience). Pudding sets, flavors meld, and your life improves dramatically.
Top and serve. Spread whipped topping evenly over the chilled cake and add your optional toppings. Slice, serve, and watch people swoon.