Shirley Corriher and her great book Cookwise. Corriher tells us the basics, which we were generally aware of already, like that fact that cake flour differs from all-purpose flour in its protein content. Cake flours usually have about 8 grams of protein per cup, or 7% - 8.5% protein total. This is much less than all-purpose which is about 10% - 12% protein. Corriher also told us some things that were surprising... Cake flour is also extra finely ground, so it gives a softer, finer texture. It also chlorinated, which explains the slightly strange smell we always find in cake flour. It's bleached with chlorine gas, and its acidity helps cakes set more quickly. It also helps it absorb water. Corriher also says that you can make softer, less rough cookies with cake flour. It really makes a marked difference in cakes, giving them a finer crumb and less rough texture. A word of caution with cake flour (and all specialty flours, actually) - make sure to check the expiration box on the box. If you're purchasing cake or pastry flour in a grocery store where there isn't a high turnover of specialty baking goods sometimes that flour is old and stale - we had a bad experience with this once. It's worth it to buy high-quality flours, especially for simple cakes and breads where their flavor really comes through. For more details on the protein content of flour see our post Good Question: Pastry Flour vs. Cake Flour. There are no perfect substitutes for cake flour, but one cake flour substitute that we have found is to replace 1/8 cup of all-purpose flour with cornstarch. In a pinch this will help, but we try to avoid using it.