Step 1: Prepare the Cake Batter

1. Preheat your oven to 350°F and grease a 9″ x 13″ baking dish.
2. In a large mixing bowl or stand mixer, combine cake mix, water, egg whites, oil, sour cream, and vanilla.
3. Mix on low speed until combined, then beat on medium speed for 2 minutes, scraping the sides as needed.
4. Pour 2 cups of the cake batter into the prepared baking dish, smoothing it evenly.

Step 2: Add the Colorful Swirl

1. Divide the remaining batter into three separate bowls (about ½ cup per bowl).
2. Add a different food coloring to each bowl and mix until well combined.
3. Using a spoon, drop dollops of the colored batter randomly over the white batter in the pan.
4. Run a butter knife back and forth through the batter to create a swirled effect. Avoid over-mixing to maintain distinct colors.

Step 3: Bake and Cool

1. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
2. Let the cake cool completely in the pan before frosting.

Step 4: Make the Frosting

1. In a large mixing bowl, beat the softened butter until light and fluffy.
2. Add salt (if using), then gradually mix in powdered sugar until fully incorporated.
3. Add vanilla extract and 2 tablespoons of heavy cream, mixing until smooth. Add more cream if needed to reach a spreadable consistency.

Step 5: Frost and Decorate

1. Spread the frosting evenly over the cooled cake.
2. Top with pastel sprinkles or Easter-themed candies for a festive touch.