Preheat the oven to 335° (use 325° if your oven doesn't go to 335°)
Prepare a 13x9 pan with a baking spray or pan release.
In the large bowl of a mixer, add all the ingredients in the order they are listed. Cake mix, flour, sugar, salt, water, egg whites, butter, sour cream, oil, and vanilla extract (add in the coffee option if using one.)
Mix the batter on low speed until it starts blending, then turn onto medium speed and mix for 2 minutes.
Stop the mixer, then scrape down teh sides and bottom of the mixing bowl and mix for 1 more minute until all blended.
Pour the cake batter into the prepared cake pan and spread the top evenly. Tap the cake pan on the counter to remove any bubbles.
Bake at 335° until a toothpick comes out clean, typically 30-40 minutes.
Remove cake from oven and allow it to cool completely.