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 bowl, combine cake mix, water, egg whites, vegetable oil, sour cream, and clear vanilla extract.
3. Mix on low speed until combined, then beat on medium speed for 2 minutes, scraping down the sides as needed.

Step 2: Color and Swirl the Batter

1. Pour 2 cups of batter into the greased baking dish.
2. Divide the remaining batter into three separate bowls (about ½ cup each).
3. Add blue, pink, and yellow food coloring to each bowl and mix until the colors are evenly distributed.
4. Drop spoonfuls of each colored batter randomly over the white batter.
5. Use a butter knife or skewer to gently swirl the colors together, creating a marbled effect. Be careful not to overmix.

Step 3: Bake the Cake

1. Place the cake in the oven and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
2. Remove from the oven and let the cake cool completely before frosting.

Step 4: Make the Frosting

1. In a large mixing bowl, beat butter until light and fluffy.
2. Add salt and mix.
3. Gradually add powdered sugar, mixing on low speed to incorporate. Scrape down the sides of the bowl.
4. Add vanilla extract and 2 tbsp of heavy cream. Mix on high speed for about 1 minute.
5. If the frosting is too thick, add 1 tbsp of heavy cream at a time until it reaches a spreadable consistency. If it’s too thin, add more powdered sugar.
6. (Optional) If you want colored frosting, divide it into bowls and mix in a few drops of food coloring.

Step 5: Frost and Decorate

1. Spread the frosting evenly over the completely cooled cake.
2. Sprinkle with pastel sprinkles or Easter candies for a festive touch.
3. Slice and serve!