To give this impressive chocolate dessert the best flavor, be sure to use Dutch-processed cocoa. It's richer in flavor and darker in color than other cocoa powders.
Place a rack in the lower third of the oven. Coat a 1- to 1-1/2-quart souffle dish with nonstick cooking spray. Add 1 tablespoon of the superfine sugar; tilt dish to coat all areas with sugar. Set aside.
In a small saucepan, whisk together cocoa powder, flour, cinnamon and milk; mixture will be thick. Over low heat, whisk in butter, then continue to cook, whisking continuously until consistency of sour cream, about 2 to 3 minutes. Remove from heat.
Place egg yolks in a small bowl. Whisk in vanilla. Whisk a heaping spoonful of the cocoa mixture into the yolk mixture to temper. Then, whisk tempered yolk mixture back into the saucepan until smooth. Transfer to a large bowl.
In a second large clean bowl with an electric mixer, whip egg whites, cream of tartar and salt until frothy. Add remaining 4 tablespoons superfine sugar, 1 tablespoon at a time, until firm, glossy peaks are formed. Fold about 1/3 of the whipped egg whites into the cocoa mixture, using a whisk to blend evenly. Gently fold in remaining egg whites.
Transfer batter to prepared dish and bake at 350°F for 22 to 25 minutes. Do not open oven door while baking, as souffle will collapse. Remove souffle from oven when top is dry, but cracks will look moist. Dust with confectioners' sugar and serve immediately.