"This über-trendy cake is actually a vintage recipe from the Deep South. It is as red as Scarlet O'Hara's dress thanks to the combination of cocoa powder and cheap liquid red food coloring. Be sure to use the cheap stuff! Fancy food coloring pastes and gels don't work nearly as well. The vinegar was traditionally added to the baking soda as a leavener. With self-rising flour, it's not necessary, but it's added here for its classic zippy flavor...."