Preheat the oven to 325°F. Line two standard (12-count) muffin pans with parchment cupcake liners and set aside.

To a large mixing bowl add the white cake mix, instant vanilla pudding mix, eggs, sour cream, vegetable oil, and whole milk. Using a handheld mixer on medium speed, blend together for 1-1½ minutes or just until the batter is smooth. The batter will be thick.

Fill each cupcake liner ⅔ full (approximately 3 tablespoons batter in each cup).

Bake for 18-22 minutes or just until a toothpick inserted into the center of a cupcake comes out with a few moist crumbs.

Allow the cupcakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely before frosting.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and salt on medium speed for 2-3 minutes or until light and fluffy. Stop and scrape the bowl and paddle as needed.

Reduce the speed to low and slowly add the powdered sugar. Once all the sugar has been added, increase the speed to medium-high and beat for another minute. Stop and scrape the bowl and paddle to ensure the butter is an even consistency.

Add the vanilla extract and 4 tablespoons of the heavy cream. Start on low, until the cream is fully incorporated, then increase the speed to medium-high and beat for 2-3 minutes or until the frosting is very light and fluffy. Scrape the bowl and paddle as needed to be sure the frosting is being evenly beaten. If needed, add the remaining 1 tablespoon heavy cream and beat again just until desired consistency.

Transfer the frosting to a large piping bag fitted with a decorative piping tip. Pipe a decorative swirl of frosting onto each of the cooled cupcakes and garnish with rainbow colored sprinkles if desired before serving.