Coat 8 (6-ounce) ramekins with cooking spray; sprinkle with 3 tablespoons granulated sugar.
Combine 1/2 cup granulated sugar, cocoa, flour, cinnamon, salt, and pepper, if desired, in a medium saucepan. Gradually add milk, stirring with a whisk until smooth. Bring to a boil over medium-high heat; cook 1 minute, stirring constantly. Remove from heat; add chocolate, stirring with a whisk until smooth. Place chocolate mixture in a large bowl; stir in vanilla, liqueur, and egg yolk.
Place the cream of tartar and egg whites in another large bowl; beat with a mixer at high speed until foamy. Add 2 tablespoons granulated sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently stir one-third of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture. Spoon evenly into ramekins; sharply tap ramekins 2 or 3 times on counter to level. Place ramekins on a baking sheet.
Bake at 375° for 30 minutes or until puffy and set. Sprinkle with powdered sugar, if desired. Serve immediately.