Can I substitute cake flour for all purpose flour?

Do I have to purchase cake flour or can I make my own using flour and cornstarch. The cake flour in the store is so expensive. Would it effect the quality of the the cake if use the homemade version.
Thank you for any advice you can share

As others have said, the main difference is the protein vs. carbohydrate content. This is most important in kneaded bread, which absolutely needs hard summer wheat with a high protein content. Cookies do not need gluten development so the protein in them is either insignificant or even a negative, so I would say cake flour in cookies would be perfectly good.

Can I substitute cake flour only instead all-purpose flour when making biscuits?

I have substituted it with no problem. May e the result is not exactly the same but it is good anyway. Do not forget to sift the dry ingredients

Because of the lower protein content of cake flour compared to AP (7% vs. 10%), I'd suggest also adding an extra egg.

It is softer and has a lower protein content but I have heard you can: To substitute cake flour for all-purpose flour use 1 cup plus 2 tablespoons cake flour for every cup of all-purpose flour

Read more: http://www.joyofbaking.com/flour.html#ixzz1kDVpBeYX

If you are just making a substitution and not trying to alter the results, you will likely need to use a touch more cake flour than you would AP flour. if the recipe calls for 2 1/2 cups AP Flour, I'd do 2 3/4 cups cake flour. If you are playing around with the recipe, trying to achieve a different goal- like a softer cookie, more delicate crumb- you will have to play around with the amount of flour. The best way to know how much cake flour to use would be to get a kitchen scale out. Weigh out a cup of AP and a cup of cake flour. Keep adding cake flour until it reaches the weight of the AP.