Preheat oven to 350F degrees. Prepare a 12x17 inch half sheet/jelly roll pan by spraying with non-stick spray then lining with parchment paper.
Whisk the flour, baking soda, and salt together and set aside.
In the bowl of stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed.
Add the sugar on medium speed and beat until fluffy and light in color, about 2 minutes.
Add in egg whites and mix until incorporated.
With mixer on low speed, slowly pour in vanilla and then sour cream. (scrape bowl if necessary)
Mix for 1-2 minutes on medium speed or until ingredients are fully incorporated.
With mixer on low speed, pour half of the flour mixture into the creamed butter mixture.
Now pour in half of the milk.
Repeat with the rest of the flour and milk and mix until combined and smooth, about 30 seconds. Spread the cake batter into the prepared pan using a small offset spatula. (be sure it is nice and level!)
Bake for 18-22 minutes or until the top is lightly browned and a toothpick inserted in the center comes out clean.
Place in freezer until ready to assemble the cake.