Place eight 6-ounce ramekins in a shallow baking pan; set aside. Preheat oven to 350 degrees F.

In a small saucepan, stir together 1/3 cup of the granulated sugar, the cocoa powder, flour, and salt. Gradually stir in milk. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir for 1 minute more. Remove from heat. Slightly beat egg yolks. Slowly add chocolate mixture to egg yolks, stirring constantly.

In a large bowl, combine egg whites, vanilla, and cream of tartar. Beat with an electric mixer on high speed until soft peaks form (tips curl). Gradually add remaining 1/3 cup granulated sugar, beating on high speed until stiff peaks form (tips stand straight). Stir about 1/4 of the egg whites into chocolate to lighten. Gently fold chocolate mixture into egg white mixture. Spoon into ramekins.

Bake about 25 minutes or until knife inserted near centers comes out clean. Serve immediately. If desired sprinkle with powdered sugar. Makes 8 souffles.