Place rack in lower third of oven and heat to 350°F. Spray 8 soufflé cups with nonstick spray. Pour 5 teaspoons granulated sugar, if using, in 1 soufflé cup. Tilt to coat bottom and sides. Tap out excess into next cup and continue until all cups are sugared.
Sift powdered sugar, cocoa and flour into the top of a double boiler. Add milk and 1/2 cup water; whisk until smooth. Cook over gently simmering water, whisking continuously, about 10 minutes, or until thickened. Remove from heat. Set aside.
Beat egg whites with cream of tartar on medium speed until soft peaks form. Gradually sprinkle in sugar, beating at high speed until stiff but not dry.
Whisk yolks and vanilla into chocolate mixture. Fold about a quarter of whites into chocolate mixture to lighten it. Fold in remaining whites. Divide evenly among the cups. It can come to about 1/4 inch below the rims.
Place cups on baking sheet. Bake at 350°F for 15 to 17 minutes, or until soufflés have puffed well above cups and toothpick inserted from the side deep into the center comes out with moist crumbs clinging to it. Do not overbake.
Sieve a little powdered sugar over each soufflé and serve immediately.