Preheat the oven according to the cake mix package directions. Grease and lightly flour your baking pan(s).
In a large mixing bowl, combine the cake mix, milk, melted butter, eggs, vanilla extract, and sour cream. Add the instant pudding mix if using.
Beat on low speed for 30 seconds, then on medium speed for about 2 minutes until smooth. Do not overmix.
Fold in any optional mix-ins such as chocolate chips, sprinkles, chopped nuts, or berries using a rubber spatula.
Pour the batter evenly into the prepared pan(s) and smooth the top.
Bake according to the package directions, checking a few minutes early. Insert a toothpick into the center; it should come out clean or with a few moist crumbs.
Let the cake cool in the pan for 10–15 minutes.
Carefully transfer the cake to a wire rack and allow it to cool completely before frosting.
If upgrading store-bought frosting, beat together the frosting, softened butter, cream cheese, vanilla extract, and a pinch of salt until light and fluffy.
Spread the frosting evenly over the completely cooled cake using an offset spatula.
Decorate with sprinkles, chocolate chips, or other toppings if desired.
Slice, serve, and enjoy your moist, bakery-style boxed cake.