For the Cake:
Preheat the oven to 350F. Prepare a 13″ x 9″ sheet cake pan by adding parchment paper. Set aside. In a standing mixer, sift in the flour, baking powder, salt and sugar. Give it a whisk and set aside. In a large bowl, add egg whites, milk, vanilla, and sour cream. Whisk together.
Attach a paddle attachment to the stand mixer. Add in room temperature butter in cubes. On medium speed, beat half of the wet mixture into the crumby dry mixture. Scrape the bowl down and add the remaining wet mixture. Beat until well combined.
Pour the batter into the prepared sheet cake pan.
Bake for 45 – 50 minutes or until a toothpick inserted comes out clean.
Let the cake cool to room temperature. Set aside.