Preheat the oven to 350 F. Have ready an ungreased 10-inch nonstick angel food cake pan. Sift together the espresso powder and flour onto a piece of parchment paper or into a bowl. Add the salt and set aside.

Put the egg whites in the large bowl of a stand mixer fitted with the whip attachment and whip on medium speed until frothy. Add the cream of tartar, increase the speed to high and continue whipping while slowly pouring in the sugar until the whites are firm and satiny, about 3 minutes.

Reduce the speed to low, add the vanilla and lemon juice and then add the dry ingredients. When the flour mixture is almost completely incorporated, remove the bowl from the mixer stand and fold in the chocolate chips with a spatula. Make sure that the chips are spread evenly throughout the batter and that the flour mixture is evenly incorporated. But be careful not to overmix, or you will deflate the batter and the cake will not rise fully in the oven.

Pour the batter into the cake pan. Cut through the batter a few times with a table knife to break up any air pockets. Bake until a skewer inserted into the center comes out clean, 40 to 45 minutes. Invert the cake and let cool completely upside down in the pan. (If the pan does not have feet, balance the inverted pan on the neck of a bottle or rest the edge of the pan rim on 3 or 4 ramekins.)

To unmold the cake, run a long knife around the inside edge of the pan. Then, holding the center tube, free the cake from the pan sides. Slip a knife between the cake and the bottom of the pan to loosen the cake and gently flip the cake over, letting it fall onto a platter. (If the pan does not have a removable bottom, release the sides with the knife, then place a platter on top of the cake. Gently invert the platter and the cake together. Lift off the pan.) To server, cut into slices with a serrated knife, using a gentle sawing motion.

Planning ahead: The cake may be made a day in advance. Wrap in plastic wrap and store at room temperature.