Combine the sifted flour, cocoa, 1/4 cup (60 ml) sugar, and optional cinnamon and sift 5 times.
Beat the egg whites until foamy and add the cream of tartar.
Continue beating until stiff but not dry, adding the remaining sugar 1 tablespoon (15 ml) at a time.
Add the vanilla.
Sift a small amount of the flour mixture over the egg mixture and fold gently.
Repeat until all the flour mixture is incorporated.
Spoon the batter into an ungreased 9-inch (23 cm) tube pan and bake in a preheated 350F (180C) oven for 45 minutes.
Remove the cake and cool it upside down, resting the tube of the pan on the neck of a bottle if the tube is not tall enough to keep the surface of the cake off the counter top.
Let the cake hang and cool for at least 90 minutes.
Remove the cake from the pan before storing.
Serve with whipped cream, ice cream, and/or chocolate sauce if desired.