I rarely ever use a recipe that calls for cake flour, but when I do it's a little frustrating as I'm not too willing to get it for just one recipe. Could I just use all-purpose as instead?
Cake flour is different to all-purpose (plain) flour as it has a much lower protein (gluten) level than all-purpose flour. Cake flour typically has a 6-8% protein content vs all-purpose which typically has 10-12% protein level. When you add liquids to flour and then mix or knead the mixture the proteins link together and form strands which can become tough. So cakes made with lower protein cake flour will have a finer, more tender crumb than those made with all-purpose flour. However, it is possible in many cases to substitute all-purpose flour for cake flour unless the cake is extremely tender (such as an angel food cake) where cake flour will give better results.
If you would like to switch to all-purpose flour and you have some cornstarch (cornflour) in your cupboard then for 1 cup cake flour (140g) measure out 1 cup (150g) all-purpose flour, remove 2 tablespoons (25g) of the all-purpose flour and replace it with 2 tablespoons (20g) of cornstarch. Whisk or sift the flour and cornstarch together before using. If you don't have any cornstarch available then just substitute 1 cup less 2 tablespoons all-purpose flour for the 1 cup of cake flour.