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I haven't tried it myself, but understand that in a pinch you can take 1 cup all purpose flour, remove 2 Tablespoons flour then replace the removed flour with 2 Tablespoons cornstarch. You might want to give that a try.
What kind of cake are you making? If you are looking to make a very fine & delicate cake you are going to need that cake (or pastry flour). However, if the cake can or does have a rustic quality (admittedly doubtful if it calls for cake flour in the first place), White Whole Wheat (1 part) and All Purpose (2 parts) will reduce the gluten content significantly allowing for better rise and is lovely for pastry where flake is important (do not use regular red whole wheat!!). Also the aforementioned corn starch sounds right, and/or a 1-part/5 or 4-part mixture of fine corn meal and all-purpose is nice with fruit-cetric cakes.
I'm no expert, but I almost always use pastry flour instead of cake flour in recipes calling for cake flour. I've never had a failure. I'm guessing, though, that if you don't have cake flour on hand, you probably won't have pastry flour on hand either. I get it from the bulk bins at my local food coop.
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