Through my own tests I’ve found that cakes made with unbleached flour always fall a little in the center about five minutes after they’re taken out of the oven. The reason for that is because butter cakes rely on flour for their structure, as opposed to a sponge cake like génoise which relies on egg foam to hold it up. When flour isn’t bleached, the individual flour particles are like little ball bearings. They can’t hold the butter in a nice emulsification and suspension, so the butter sinks to the bottom. That creates a texture that’s awful and pasty, and it affects the flavor, since as you know in baking, if the texture is off it completely changes the taste.
Also where cakes are concerned you get a better flavor with bleached flour, a more melt-in-the-mouth flavor. Chlorine bleach, it’s said, enhances a cake’s flavor. I found in my own tests when I was writing The Cake Bible that it gives a cake a more flowery and sweet flavor, and when I say “sweet” I don’t mean sugary sweet, I mean lovely.