Step 1: Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking pan with non-stick spray or butter.
Step 2: Prepare the white cake mix according to the package instructions, using egg whites to ensure the cake remains as white as possible. Pour into the prepared pan.
Step 3: Bake the cake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and let it cool on a wire rack for 15 minutes.
Step 4: Using the handle of a wooden spoon, poke holes across the entire surface of the warm cake, spaced about 1 inch apart.
Step 5: Divide the sweetened condensed milk into 4 small bowls. Add a drop of different gel food coloring to each bowl and stir until vibrant.
Step 6: Carefully pour or spoon the colored milks into the holes of the unicorn poke cake, alternating colors to create a rainbow effect.
Step 7: Cover the cake and refrigerate for at least 4 hours, or ideally overnight, to allow the milk to soak into the crumb.
Step 8: Spread the thawed whipped topping evenly over the top of the chilled cake. Garnish generously with sprinkles and serve immediately.