A.P. vs. Cake & Pastry FloursActually, all-purpose flour is a blend of hard and soft wheat flours. Soft wheat flour is generally the better choice for baked goods that do not need to rely upon a high gluten content -- most biscuits, pastries, and cakes, for example. On the other hand, when you use a.p. flour for most cakes & other delicate baked goods, their texture can be toughened because of too much gluten. If something is best made with a blend of the two flours, as is the case with popovers, then you can create a suitable mixture.
Cake flour is milled from the soft winter wheat. The gluten content is low, which makes it generally unsuitable for yeast breads. However, a small proportion of soft wheat flour can be added to the bread flour in order to improve the flavor of certain water breads. (I like to make French bread using a mixture of ¾ bread flour to ¼ pastry flour; it’s not strictly necessary to use the pastry flour for good Fr. bread, but I’m convinced that the flavor is closer to the real thing when it is included.)
Nancy Silverton, in the prefatory notes to her book, Desserts, wrote: "[My recipes] are made with unbleached pastry flour, unless otherwise specified. Cake flour is an acceptable substitute. Both these flours are made of 'soft' wheat, which contains little gluten, the protein that can make a pastry tough. Cake flour contains even less gluten than pastry flour. Products made with soft flour will be more tender than if they are made from all-purpose flour....If you aren't sure what kind of flour is in an unmarked bag or jar, just squeeze a handful -- cake flour and pastry flour will hold their shape." (p. 23)
Addendum: A good recipe for an 8-inch Red Velvet cake is included in "food archaeologist" Susan Waggoner's aptly titled Little Cakes -- a slim (fewer than 100pp.) volume published last year. I also drew out my copy of The Magnolia Bakery Cookbook, suspecting that a recipe for this cake would have been listed...but no!
As a curiosity: Could red crystalline decorating sugar be used effectively in Red Velvet cake? It provides a stiking contrasting effect in pound cake.
"A house is beautiful, not because of its walls, but because of its cakes." ~ Old Russian proverb