Preheat the oven to 325°F. Line two cupcake pans with patriotic paper liners.
In a large bowl, combine the cake mix, eggs, melted butter, milk, sour cream, and vanilla extract. Mix just until combined and smooth. Do not overmix.
Fill each cupcake liner about two-thirds full.
Bake for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean. Cool completely before frosting.
For the frosting, beat the butter until light and fluffy. Add the powdered sugar one cup at a time. Mix in the vanilla extract and enough heavy cream to create a smooth, pipeable consistency.
Transfer the frosting to a piping bag fitted with a 1M tip and pipe swirls onto the cooled cupcakes.
Finish with patriotic sprinkles and American flag cupcake toppers.