While you can make cakes with regular flour, using plain flour instead gives the final product a lighter, more spongelike texture. This is due to the lower protein content of plain flour. It contains only 6% to 8% gluten as opposed to regular flour's 10% to 12% gluten. If your supermarket does not carrysell plain flour, or if you only make cakes occasionally, you can make the plain flour yourself.
Put the flour and cornflour into a bowl. Mix well with a fork to blend.
Pour the flour and cornflour mixture into a flour sifter. Sift the mixture. This will help blend the two ingredients together thoroughly.
Use the mixture as if it were plain flour. You do not need to make any other modifications or substitutions.
This will make 128 g (1 cup) of plain flour. If you want to make more or less, scale the all-purpose flour and cornflour accordingly.