Preheat the oven to 350°F and adjust the racks to ensure the cake sits in the center of the oven.
Ensure all equipment is completely grease-free by thoroughly scrubbing even clean items, as any residue will ruin the cake.
Sift the powdered sugar, flour, and cocoa together three times and set aside.
Combine the egg whites, cream of tartar, vanilla, and salt in a mixing bowl.
Beat the mixture at high speed.
Gradually add the sugar in a slow, steady stream once the egg whites become foamy, keeping the beaters running.
Continue beating on high speed until stiff peaks form.
Gently fold the dry ingredients into the egg whites 1/2 cup at a time, taking care not to stir.
Add the next 1/2 cup of dry ingredients once the previous addition is halfway incorporated.
Repeat the process until all dry ingredients are fully incorporated.
Gently spoon the batter into an ungreased 10-inch two-piece tube pan.
Run a sharp knife in a circle through the batter to level it and remove large air bubbles without deflating the mixture.
Bake at 350°F in the center of the oven for 35 minutes.
Immediately invert the pan onto a wire rack to cool after removing it from the oven.
Leave the cake in the pan to cool completely while inverted.
Turn the pan upright once cooled and run a sharp knife around the outer and inner edges.
Remove the outer ring of the pan and then carefully detach the cake from the center piece.
Slice the cake with a serrated knife and serve with fresh fruit and whipped cream.
