I'm going to bake a chocolate cake (for a layer cake) and I keep butter on hand, but not vegetable shortening. Can I make the substitution without much of a problem? I'm already subbing "homemade" cake flour (the AP-cornstarch sub) for the regular cake flour. I'd rather not buy ingredients that I won't use much (i.e. cake flour, veg. shortening)--they take up valuable space.