Preheat oven to 325 degrees F.
Butter and flour two 9-inch round cake pans or 3 8-inch round cake pans.
In a medium bowl, whisk together flour, salt, baking powder, and baking soda.
In a large bowl, cream butter and sugar together until light and fluffy.
Add egg whites, one at a time, beating after each addition.
Beat in vanilla.
Add sour cream, and beat for 30 seconds.
Add flour mixture in three additions, beating to blend after each addition.
Divide batter into prepared pans.
Bake cake until a toothpick inserted into the center comes out clean, 50-60 minutes.
Remove from oven and cool for 10 minutes.
Run a sharp knife around the edges of the pans, then turn cakes out onto a rack to cool completely.
If using two 9-inch cakes, take a serrated knife, and slice each of the two cakes in half horizontally. If using three 8-inch cakes, as I did, there is no need to slice any of them in half.
Place first layer of cake on a cake plate.
Spread a thick layer of frosting over the first cake layer.
Slice strawberries, leaving enough unsliced to decorate the cake.
Arrange sliced strawberries all over the frosting in a single layer.
Place the next cake layer on top and press down gently so that the strawberries are firmly set into the frosting.
Frost this second cake layer and repeat laying sliced strawberries on top of the frosting.
Repeat until the top cake is finally placed.
Generously frost the entire cake on top and sides, beginning with spreading two cups of frosting over the top and sides of the cake in a thin layer, then frost with remaining frosting.
This recipe makes a very big 4-layer cake or a thick 3-layer cake!