Substitute for Cake Flour (and When It Matters)

I was making some scratch confetti cupcakes for my daughters' birthday yesterday, using a white cake recipe and some pastel cake/cookie sprinkles mixed in with the batter. (That's them above, iced with a buttercream frosting and topped with pastel M&Ms.) My white cake recipe calls for cake flour, but what I have is all-purpose flour. 

What is cake flour and why and when does it matter?

Cake flour is a refined wheat flour that is made with a lower protein wheat than all-purpose or bread flour. It's the protein (gluten) that forms stretchy bonds in bread dough -- nice for holding a piece of bread together but makes for a slightly dense cake. Cake flour makes a finer cake with a smaller crumb. For most scratch cakes, the difference in end result is minimal between using all-purpose or using cake flour. (And that's why I don't keep cake flour on hand, but instead use a substitute when it might make a difference.) Some cakes, like a light white cake or angel food cake do turn out lighter using cake flour.

So, is there a good substitute?

There sure is -- corn starch (or cornflour, but not cornmeal). Using corn starch in place of a small amount of all-purpose flour reduces the total grain protein content and makes a lighter crumb. Some shortbread-type cookies call for cornstarch in addition to flour. The end result is a more tender cookie than a similar cookie using flour only.

The cornstarch plus all-purpose flour substitute for cake flour

1 cup of cake flour equivalent can be made from 2 tablespoons (or 1/8 cup) cornstarch plus 7/8 cup of all-purpose flour. 

For every cup of cake flour called for in a recipe: measure 2 tablespoons of cornstarch into a 1 cup dry measuring scoop. Then add all-purpose flour to fill the 1 cup scoop. Increase or decrease according to this ratio. So, if a recipe calls for 1  1/4 cups of cake flour (which is what my 10 cupcake recipe calls for), measure 2  1/2 tablespoons of cornstarch into a measuring cup, then fill with all-purpose flour to 1  1/4 cups. 

FYI, arrowroot can be used in place of cornstarch, for those with sensitivities to corn.

Little cakes topped with chocolate candies -- always a hit with my family!