Preheat oven to 350 degrees.
Grease (4) 4x10x¾ inch pans. These are a specialty pan made by Wilton and available from Amazon. I will include the link at the bottom that you can use to earn an affiliate commission. Alternatively, a large loaf pan can be used and the cake sliced in four layers horizontally. Combine cake flour, salt, baking soda and baking powder and set aside.
In a separate large mixing bowl, mix butter and sugar on medium speed until smooth.
Add egg whites and mix on high until well combined.
Add vanilla and milk and mix well.
Add dry ingredients to the wet ingredients, mixing well on low until combined.
Fill pans, stopping about ¼ inch from the top of pan. Place pans on center rack of preheated oven and bake for 15-20 minutes or until toothpick inserted in center comes out clean. (If you choose to use a single pan, the baking time will be longer, more like 40 minutes or so, until the toothpick comes out clean.)
Remove to a cooling rack and cool completely Meanwhile, prepare the icing.