Preheat the oven to 325 degrees Fahrenheit. Line 24 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 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