01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter according to package directions. Pour into prepared pan and bake for 25-30 minutes, or until a wooden toothpick inserted in center comes out clean.
03 - Remove cake from oven and allow to cool in pan for 10 minutes. The cake should still be warm for the next step.
04 - Using the handle of a wooden spoon, poke holes every 1-2 inches across the entire surface of the warm cake, creating a grid pattern.
05 - In a large mixing bowl, whisk together both packages of instant vanilla pudding mix with cold milk for about 2 minutes until mixture begins to thicken slightly.
06 - Divide pudding mixture evenly among 4 separate bowls. Add gel food coloring to each bowl—one pink, one purple, one blue, and one yellow. Stir each until vibrant color is achieved.
07 - Spoon the colored puddings randomly into the holes across the cake, alternating colors to create a rainbow effect. Gently press pudding into each hole.
08 - Spread thawed whipped topping evenly over the entire surface of the cake, covering all pudding-filled holes completely.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping while it's still slightly sticky.
10 - Refrigerate cake for at least 1 hour to allow pudding to set and flavors to meld. Slice into 12 squares and serve chilled.