Blackberry Jam Cake

This was a standard of the 19th-century American baking repertoire and deserves to be better known again. I came across the recipe so many times during a several month period—mostly in 19th- and early 20th-century handwritten manuscript cookbooks—that my curiosity got the better of me and I sat down with as many versions of the recipe as I could find and compared them. Only the more recent versions called for the cocoa, which imparts a very subtle flavor and a bit of color to the batter. As