Technically, cake flour does not have “ingredients”. It is a finely ground flour that has a lower protein content which makes for lighter and fluffier cakes. It is possible to make a cake flour substitute by sifting together 1 cup of flour and 2 tablespoons of cornstarch.
When making a cake flour substitute, the goal is to remove some of the heavy gluten and replace it with a product that makes the cake lighter and more tender. Cornstarch itself is not a tenderizer. Cornstarch works with the cake’s other ingredients to reduce the development of gluten. Regular all-purpose flour takes the liquids from a recipe, such as eggs and buttermilk, and absorbs them.
Working that liquid-filled flour in a mixer causes more and more gluten to form. Enough gluten leads to crusty, dense bread instead of a light, fluffy cake. The cornstarch in a homemade cake flour substitute competes with the flour and absorbs some of the liquid itself, which means the flour has less liquid to help it develop tough gluten. Ideally, when making recipes calling for cake flour, the baker should utilize store-bought cake flour to get the correct amount of protein needed for tender, light, fluffy cakes.