For the Cake:
Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.In a large mixing bowl, combine the cake mix, water, egg whites, vegetable oil, sour cream, and vanilla extract. Use an electric mixer on low speed until everything is just combined. Scrape down the sides of the bowl.
Increase the mixer speed to medium and beat for 2 more minutes, scraping the bowl occasionally to ensure a smooth batter.
Pour about 2 cups of this plain white batter into your prepared baking pan, spreading it evenly.
Divide the remaining batter equally among three separate medium bowls (you'll have about 1/2 cup in each).
Add a few drops of blue food coloring to one bowl, pink to another, and yellow to the last. Gently stir each bowl until you reach your desired pastel shades.
Now, randomly drop spoonfuls of each colored batter over the white batter in the pan. Don't worry about perfection!
Using a butter knife, gently run it back and forth and in circular motions through the colored batter to create beautiful tie-dye swirls. Be careful not to over-mix, as you want distinct colors.
Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Let the cake cool completely on a wire rack before you even think about frosting it!
For the Buttercream Frosting:
In a mixing bowl with an electric mixer, whip the softened unsalted butter for 1-2 minutes until it's noticeably lighter in color and fluffy.Gradually add the powdered sugar and salt to the whipped butter. Beat on low to medium speed for about 1 minute until well combined, scraping down the sides of the bowl as needed.
Stir in the vanilla extract and 2 tablespoons of heavy cream. Beat for another minute until smooth.
If your frosting is too thick, slowly add more cream, one tablespoon at a time, until it reaches your desired light and spreadable consistency. If you want to color your frosting, add a few drops of food coloring now and mix well.
Once the cake is completely cool, spread your fluffy homemade buttercream frosting generously over the top.
Finish by decorating with your favorite sprinkles for that extra festive touch!