01 - Preheat oven to 350°F. Generously coat a 9x13-inch baking pan with non-stick cooking spray, ensuring complete coverage to prevent sticking.
02 - Prepare cake batter according to package instructions. Pour into prepared pan and bake for 25-30 minutes until a toothpick inserted into the center comes out clean.
03 - Remove cake from oven and let cool in pan for exactly 10 minutes. The cake should be warm but not hot for the poking process.
04 - Using the round handle of a wooden spoon, poke holes every 1-2 inches across the entire surface of the warm cake, pressing about halfway through the depth.
05 - In a large mixing bowl, whisk together both vanilla pudding mix packages with cold milk for 2 minutes until mixture begins to thicken slightly.
06 - Divide pudding mixture evenly among 4 separate bowls. Add a few drops of gel food coloring to each bowl—one pink, one purple, one blue, one yellow—mixing until vibrant and consistent in color.
07 - Spoon the colored puddings randomly into the holes, alternating colors to create a rainbow effect. Gently tap pan on counter to settle pudding into holes.
08 - Spread thawed whipped topping evenly over the entire surface of the cake using an offset spatula or back of a spoon, creating swirls for visual interest.
09 - Immediately sprinkle unicorn or confetti sprinkles generously over the whipped topping while it's fresh so they adhere properly.
10 - Refrigerate cake for at least 1 hour to allow pudding to set completely. Slice into 12 rectangular portions and serve chilled.