Preheat the oven to 375ºF.

In a medium mixing bowl, whisk together the flour, cocoa powder, and 1 cup sugar.

Place the egg whites, cream of tartar, and salt in the bowl of an electric mixer fitted with a whisk attachment and beat on high until stiff peaks form. Gradually beat in the remaining ¾ cup sugar.

Turn the mixer to low and beat in the flour mixture and vanilla. Scrape down the sides and bottom of the bowl to ensure all the flour is incorporated.

Transfer the batter to an ungreased angel food cake pan and smooth out the top. Run a dinner knife through the batter to remove any air pockets. Bake for 35 minutes. Let the cake cool upside down until completely cooled. (If your angel food cake pan does not have cooling legs, place the center of the pan upside down over the neck of a bottle to cool.)

Run a dinner knife around the rim and center of the cake pan, invert on a serving platter, and tap the bottom of the pan to release the cake.

Place the chocolate chips in a heat-proof bowl. Heat the cream over medium heat until it begins to simmer. Pour the cream over the chocolate chips and let stand for 5 minutes. Stir until smooth and glossy. Pour the chocolate over the top of the cake, letting some chocolate drizzle down the sides. Slice and serve.