I'm confused about the difference between pastry flour and cake flour? Can I use them interchangeably?
It all has to do with protein.
Pastry flour contains 8.5% - 9.5% protein, which is low relative to all purpose (10% - 12% protein) and bread flour (12% - 13% protein). It is used in delicate cakes and pastries, pie crusts, cookies and muffins. . Absorbs less liquid in recipes. It is from soft red winter or soft white winter wheat for use in biscuits, pancakes, pie crust, cookies, muffins and brownies, pound and sheet cakes.
Pastry flour is found in most grocery stores. We like Bob's Red Mill Pastry Flour and Arrowhead Mills Pastry Flour. According to Baking 911, you can mix 1 cup of cake flour and 2 cups of all-purpose flour and get a good close protein mix to use for pastry flour, although this is not always a completely reliable substitution.
Cake flour generally has 7% - 8.5% protein. It is beached, and it is the bleaching process that weakens the proteins further and results in a more delicate flour, which results in more delicate products like chiffon and angel food cakes. Cakes that have a high ratio of sugar and liquid to flour are best made with cake flour, whose bleached protein molecules are able to carry more than its weight.
Unlike other flours, cake flour is usually found in a box. Common brands are Swans Down and Softasilk. According to Baking 911, a sometimes-reliable substitution for 1 cup of cake flour is 1 cup all-purpose flour (preferably bleached) minus 2 tablespoons, and then add in 2 tablespoons cornstarch.
For most baking, a good quality all-purpose flour is a fine substitute for either of these flours, you may just experiment by decreasing the quantities a few teaspoons per cup called for and see if that results in a tender consistency.