Preheat the oven to 350 degrees F. Line three 6-inch cake pans with parchment paper and grease the sides.
Combine the cake mix, buttermilk, oil, sour cream, eggs and egg whites in a large bowl and beat with an electric mixer for 2 minutes on medium speed.
Divide the cake mix into three even portions in separate bowls. Add a few drops of blue food coloring to one bowl and mix until well combined. Repeat the process with the remaining batter with the pink and yellow food coloring.
Pour the batter into the prepared cake pans (one color per pan) and bake for 22-27 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pan for 10 minutes before turning out onto a wire rack to finish cooling.
Once the cakes have cooled completely, prepare the buttercream. In a large bowl, beat the butter with a hand or stand mixer until smooth and creamy.
Add the vanilla, salt and powdered sugar 1 cup at a time, beating well after each addition until smooth and creamy. If needed, add milk 1 teaspoon at a time until the desired consistency is reached.
Level the cakes with a cake leveler or serrated knife. Place one cake on a cake stand and spread 1/2 cup of frosting over top. Add a second cake layer and top with another 1/2 cup of frosting. Add the third layer of cake and apply a thin crumb coat of frosting to the top and sides of the cake. Freeze the cake for 30 minutes or until the crumb coat is firm.
Add 3/4 cup of butter cream to a piping bag fitting with a large star piping tip of your choice and set aside.
Frosting the top and sides of the cake with the remaining butter cream. Pipe rosettes on top of the cake and place a candy coated egg on top of each rosette.
Slice and enjoy!