Prepare cake according to box instructions in a 9 x 13" pan.: The kitchen fills with warm, chocolatey scent as you mix and pour the batter into the pan, and that smell is a reliable sign that things are going right. Use a bowl big enough to combine the cake mix , eggs , oil , and water without splashing, and scrape the sides so all dry ingredients are incorporated. Your batter should be smooth and shiny, not lumpy. A common mistake is overbeating which develops too much gluten, leading to a tougher crumb, so combine until just uniform. When you pour the batter, tap the pan lightly to release any large air bubbles, which helps ensure an even rise and a flat top that is easier to poke later.
Right before cake comes out of the oven prepare the pudding by adding the chocolate pudding mix, milk, and peppermint extract in a large bowl. Whisk to combine (2 minutes). Add 1 cup of the peppermint crunch baking chips and stir together.: You will notice the aroma of chocolate intensify as the pudding thickens, and the coolness of the cold milk against your wrist is a small comfort while whisking. Whisk briskly but steadily for about two minutes until the mixture is glossy and begins to thicken, which signals the gelatinized mix doing its job. Stirring in one cup of the peppermint chips creates little crystalline pockets that will give texture once the pudding soaks into the cake. A frequent error is preparing the pudding too early which causes it to firm up before it reaches the warm cake; timing it to be ready as the cake exits the oven ensures it can flow into the holes properly.
After cake comes out of the oven use the back of a wooden spoon to poke holes all over the cake.: The steam rising from the warm cake smells rich and toasty when you begin poking, and the sound is soft as the spoon meets tender crumb. Make holes spaced evenly across the surface, pressing down to the bottom without cutting through the pan. This creates channels for the pudding to penetrate so each bite is infused. One troubleshooting tip is to avoid poking too shallowly which prevents the pudding from sinking in, or too deeply which can tear the cake; aim for consistent depth that reaches the base but keeps the cake intact.
Pour the pudding mix over the warm cake and use a rubber spatula to spread it evenly over the cake, making sure to get the pudding into the holes.: As you pour, the pudding will hiss slightly on the warm surface and begin sinking into the holes, carrying those pockets of peppermint chips with it. Use a rubber spatula to coax the pudding across the surface, pressing gently so it seeps into each channel. You want the top covered evenly, but not flooded, so the cake remains moist without becoming soupy. A common misstep is pouring too much at once which can pool on the surface rather than absorb, so pour steadily and use the spatula to help it settle.
Refrigerate until chilled completely (about 1 hour).: Chilling transforms the pudding from glossy and fluid to set and cohesive, and the cake firms up so it can be sliced cleanly. Place the pan on a level shelf in the fridge and avoid stacking anything on top while it sets. I like to check after about an hour, pressing gently to ensure the pudding is no longer loose in the middle. Rushing this step is the most common error, because cutting too early will cause the topping to slide and the slices to be untidy.
Top evenly with Cool Whip and sprinkle with the remaining 1 cup of Peppermint Chips.: The Cool Whip adds a pillowy contrast to the dense, pudding soaked cake beneath, and smoothing it creates a pretty presentation that invites slicing. As you spread, the cool, creamy texture is a relief after the warm assembly steps. Scatter the remaining peppermint chips so each slice gets a crunchy, minty finish. A typical mistake is waiting too long to add the topping which allows moisture to form on the cake surface; add the Cool Whip when the cake is fully chilled for the best hold.
Enjoy!: When you serve a slice, you should see glossy pockets where pudding soaked into the holes and a light, bright topping flecked with peppermint crunch. The first bite gives a pleasing contrast between soft cake and icy chips, with a cool mint finish. Plate it chilled, and savor how the textures and flavors play together. One tip is to slice with a warm knife for cleaner cuts, but avoid letting the slices warm too long on the counter which can soften the topping excessively.
