I did not understand why there is baking soda in your recipe for White Cake.
This is the way I do it:
Beat butter and sugar until creamy. Beat in vanilla extract.
Sift together flour, salt, and baking powder.
Alternatelly add the sifted dry ingredients and milk to the butter-sugar mixture. Beginning and ending with flour.
Beat the whites, and with a rubber spatula, gradually fold the whites into the batter.
My white cake always comes out nice and fluffy.
Norma