In a very large mixing bowl allow the egg whites to stand at room temperature for 30 minutes.
Meanwhile, sift the powdered sugar, flour, cocoa powder, and cinnamon together 3 times. Set aside.
Add the cream of tartar and vanilla to the egg whites. Beat with an electric mixture on medium speed until soft peaks form (tips curl).
Gradually add granulated sugar, about 2 tablespoons at a time, beating on high speed until stiff peaks form (tips stand straight).
Sift about one-fourth of the dry mixture over the beaten egg whites; fold in gently. Repeat, folding in the remaining dry mixture by fourths. Pour into and ungreased 10-inch tube pan. Using a narrow metal spatula or knife, gently cut through the batter to remove any large air pockets.
Bake in a 350 degree F oven on the lowest rack for 40 to 45 minutes or until the top of cake springs back when lightly touched.
Immediately invert cake ( leave in pan); cool 3 hours or until completely cooled. When cool, loosen sides of cake from pan. Remove cake from pan. To serve, slice into wedges. If desired, serve cake drizzled with chocolate-flavored syrup and garnish with strawberries.