Preheat oven to 325F. Line 36 wells of muffin pans with cupcake liners; set aside.
In a large mixing bowl, whisk together the dry cake mix, flour, sugar, and salt.
Add the water, sour cream, egg whites, vegetable oil, and vanilla. Beat the mixture with an electric mixer on low speed just until the ingredients are combined.
Scoop the cake batter into the prepared muffin pans, filling each to about ¾ full.
Bake the cupcakes at 325F for 18 minutes until a toothpick inserted into the center comes out clean.
Cool in the pan for 2-3 minutes, then transfer to a wire cooling rack to come to room temperature before frosting.
For The Buttercream Frosting
In a large mixing bowl, beat the butter with anelectric mixer on medium speed until smooth.
Beat in the powdered sugar on low speed 1 cup ata time until combined.
Add the vanilla extract and heavy cream thenbeat until light and fluffy.
Spread or pipe the frosting onto the completelycooled cupcakes and decorate with sprinkles if desired.
Notes
Cake flour can be used instead of all-purpose.
For a more traditional “wedding cake” flavor, add ¼ teaspoon almond extract. Now, that may not seem like much, but almond extract is very strong and the idea is just to add a hint of the flavor that doesn’t overpower the vanilla.
It’s best to use full-fat sour cream. Plain Greek yogurt could be used instead, though sour cream works better.
Keep leftover cupcakes in an airtight food storage container and refrigerate for up to five days. Bring to room temperature before serving.