Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour a 9x13 inch baking pan. This ensures the cake won't stick.
Step 2: Prepare the white cake mix according to package directions, typically by combining the mix with eggs, oil, and water. Mix until the batter is smooth.
Step 3: Pour the cake batter evenly into the prepared baking pan. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake cool in the pan on a wire rack for about 20 minutes.
Step 4: Once the cake has cooled slightly, use the handle of a wooden spoon or a round chopstick to poke holes evenly over the entire surface of the cake. Make sure the holes go almost all the way to the bottom but don't pierce through. Aim for holes about 1 inch apart.
Step 5: In a small bowl, whisk together the sweetened condensed milk with 1/4 cup of water to thin it slightly. Gently pour this mixture over the entire cake, allowing it to seep into the poked holes.
Step 6: Prepare each color of gelatin separately. In individual bowls, dissolve one 3 oz package of gelatin in 1 cup of boiling water. Stir until completely dissolved, then let each color cool for about 10-15 minutes at room temperature (it should still be liquid but not steaming hot).
Step 7: Carefully spoon different colors of gelatin into different holes across the cake, creating a random or patterned rainbow design. Continue until all holes are filled with various colors.
Step 8: Once all the gelatin is poured, carefully transfer the cake to the refrigerator and chill for at least 4 hours, or preferably overnight, until the gelatin is completely set.
Step 9: Once the gelatin is fully set, spread the thawed whipped topping or homemade buttercream evenly over the top of the chilled cake.
Step 10: Decorate your unicorn poke cake with rainbow sprinkles, edible glitter, or any fun unicorn-themed decorations you like.
Step 11: Slice, serve, and enjoy the magical layers of your colorful creation!