Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: Prepare the white cake mix according to package directions. For a richer cake, use milk instead of water and add an extra egg yolk.
Step 3: Pour the batter into the prepared pan and bake for 28-32 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 4: Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for about 15-20 minutes.
Step 5: While the cake is still warm (but not hot), use the blunt end of a wooden spoon or a large straw to poke holes all over the top of the cake, about 1 inch apart. Don't go all the way through to the bottom.
Step 6: In three separate bowls, prepare each gelatin mix. Dissolve one packet of gelatin in 1 cup of hot water (follow 'jiggly' directions on the box for best setting results). Stir until completely dissolved.
Step 7: Carefully pour different colors of gelatin into alternating holes across the cake. Aim for a vibrant rainbow pattern.
Step 8: Gently pour the entire can of sweetened condensed milk evenly over the entire cake, ensuring it seeps into all the holes and saturates the cake.
Step 9: Place the cake in the refrigerator for at least 2-3 hours, or preferably overnight, to allow the gelatin to set and the milk to fully absorb. This chilling time is essential.
Step 10: Once the cake is completely chilled and set, spread the thawed whipped topping evenly over the top, using an offset spatula for a smooth finish.
Step 11: Decorate generously with assorted rainbow sprinkles, edible glitter, mini marshmallows, or other unicorn-themed embellishments.
Step 12: For an ultimate unicorn look, you can add fondant or candy melt 'horn' and 'ears' to the top of the cake. Place them just before serving.
Step 13: Return the finished cake to the refrigerator until ready to serve. This helps the topping stay firm.