For one cup of cake flour put two tablespoons of cornstarch in the bottom of your measuring cup and then fill with regular AP flour, sift well.
I have always thought that cake flour was simply a finer grind of flour than regular flour. That being the case, one cup of cake flour is likely going to weigh a little more than regular flour because it will actually be more flour. That is why cake flour absorbs more liquid.
I know that when I do the opposite, when I need cake flour but do not have it, I put the normal flour through the sifter a few times and the consistency of the cake is the same.
But that is not the case...
Cake flour is made from a lower protein, softer flour than AP or bread flours. The result is a softer fluffier baked good.
As I stated in my earlier post, higher protein flour absorbs more liquid than lower protein cake flour so the adjustment I suggested.
Bleached cake flours are especially formulated to absorb high levels of sugar, eggs and water/milk. This produces a fine crumbed, higher-rising, lighter cake than one made with all-purpose.
Recipes utilising cake flour are formulated to take the characteristics of the chemically adaped flour.
All purpose + starch will substitute for cake flour but the chemistry is not the same and some recipes will fail.