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["Oven to 350 degrees.", "Pour the vinegar into the bowl of your mixer, and using a paper towel, wipe down the entire bowl with the vinegar, then wiping the bowl dry with the towel.", "Use the vinegar soaked towel to wipe down your whip attachment, too, and attach to the mixer.", "Sift together the espresso powder, cake flour, and salt.", "Set aside.", "Put the whites into the mixer bowl.", "Whip the whites on medium speed until frothy, then increase the speed to high and slowly start to add the granulated sugar a little at a time until it is all incorporated.", "Continue to whip the whites until they are stiff but still shiny- about 3-4 minutes.", "Egg whites that are room temperature will whip far better than cold ones.", "Stop the mixer and remove the bowl.", "Add the lemon juice and vanilla and fold very gently into the batter.", "Start adding the flour mixture, folding very gently, until all of it has been added, then add the chocolate and gently fold in until the chips are spread through the batter and the flour is evenly mixed into the whites.", "Do not overmix.", "Pour the batter into 10"" angel food cake pan, ungreased and cut rhrough it a few times with a butter knife to break any air pockets.", "Bake about 40-45 minutes, or until a skewer insetted in the middle comes out clean.", "Invert the cake on a rack and let it cool upside down in the pan.", "Unmold the cake, let cool.", "Slice with a serrated knife.", "Excellent served with hot fudge sauce and a sprinkle of cocoa powder."]
