Heat the oven to 350 degrees. Grease and flour 2 8-inch round baking pans. In a large bowl, stir the flour, sugar, baking powder, baking soda and salt with whisk until blended. Add 1 cup sour cream, 1/2 cup butter, the egg whites and 1 teaspoon vanilla. Beat on medium speed of an electric mixer for 2 minutes until light and fluffy. Pour into the pans and bake for 22 to 30 minutes or until a toothpick inserted in the center comes out clean. Cool in the pans for 10 minutes. Remove from the pans to a cooling rack. Cool completely (about 1 hour). In a medium bowl, beat 1/3 cup sour cream and 1/4 cup butter on low speed of an electric mixer until blended. Beat in the powdered sugar and 1/2 teaspoon vanilla until smooth and spreadable. On a serving plate, place 1 cake layer. Frost the top of the cake layer with about 1/3 cup frosting. Top with the second layer. Frost the side and top of the cake with the remaining frosting. Refrigerate leftovers.