Coat 8 (6-ounce) ramekins with cooking spray; sprinkle with granulated sugar. Place on a baking sheet; set aside.
Combine the milk, 3/4 cup powdered sugar, cocoa, flour, and salt in a medium, heavy saucepan, and cook 5 minutes over medium heat, stirring constantly with a whisk until mixture thickens and comes to a boil. Remove from heat; add chocolate, stirring until melted. Stir in liqueur. Gradually add chocolate mixture to egg yolks in a bowl, stirring well. Return mixture to pan. Cook 1 minute over medium heat, stirring constantly. Spoon mixture into a large bowl; cool to room temperature, stirring occasionally.
Preheat oven to 375°.
Beat the egg whites and cream of tartar at high speed of a mixer until foamy. Add 1/3 cup powdered sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently stir one-fourth of the egg white mixture into the chocolate mixture; gently fold in remaining egg white mixture. Spoon the batter evenly into prepared ramekins. Bake at 375° for 23 minutes or until puffy and set. Sprinkle the souffles with 1 tablespoon powdered sugar. Serve immediately.