Special equipment: 10-inch angel food cake pan, ungreased
Preheat the oven to 350 degrees F.
Sift together the espresso powder, cake flour, and salt. Set aside.
Put the egg whites in the bowl of an electric mixer. Using the whisk attachment, whip the egg whites on medium speed until frothy. Add the cream of tartar. Increase to high speed and slowly pour in the sugar. Continue whipping until the whites are stiff but still shiny, about 3 minutes.
Reduce to low speed and add the vanilla extract and lemon juice. Add the reserved flour mixture. When the flour mixture is almost completely incorporated, remove the bowl from the machine and fold in the chocolate chips. Make sure that the chocolate chips are spread throughout the batter and that the flour is evenly mixed into the egg whites. Do not over mix.
Pour the batter into the prepared pan and cut through it a few times with a dull knife to break up any air pockets.
Bake until a skewer inserted in the middle comes out clean, 40 to 45 minutes. Invert the cake on a rack and let it cool upside down in the pan.
Unmold the cake and slice it with a serrated knife.
This cake will keep for several days. Store wrapped in plastic wrap at room temperature.