Cake flour isn’t just a very low-protein (gluten) version of all-purpose flour, it’s actually made from a completely different species of wheat known as club wheat. The wheat is cracked, sifted and very finely milled to an almost talcum powder-like consistency, making it quite light by volume (about half an ounce less per cup than all-purpose flour).
Of course cake flour is usually quite heavily bleached. That obviously what’s responsible for the whiteness of cake flour, though the bleaching also imparts some other very important characteristics. For one, it helps make the starch granules more absorbent (especially in very sugary batters), increasing their ability to form the gels that hold a cake layer up. Bleaching also helps fat molecules adhere more readily to starch granule surfaces, resulting in better fat distribution. The cumulative effect is lightness, sweetness, richness and tenderness…all the attributes one seeks in a good cake.
One side effect of the heavy bleaching that some people notice is a slightly acrid smell or sour taste. The reason for that is a trace amount of hydrochloric acid that the processing leaves behind.
Given what cake flour is made from and how it’s treated, a concocted equivalent is by no means ideal. However you can approximate one cup of cake flour in the following way: start with one cup of all purpose flour, subtract two tablespoons, and replace them with corn starch (corn flour).