Combine milk, chocolate, and stick cinnamon, if desired, in a heavy medium saucepan. Cook, stirring occasionally, over medium heat until mixture simmers and chocolate is melted. Remove saucepan from heat and set aside.
Meanwhile, cook the 1/3 cup sugar in a heavy 8-inch skillet over medium-high heat until sugar begins to melt; shake skillet occasionally. Do not stir. When sugar starts to melt, reduce heat to low; cook 5 minutes or until all sugar is melted and golden, stirring with wooden spoon. Immediately divide among four 6-ounce custard cups or ramekins; tilt to coat bottoms. Let stand 10 minutes.
Lightly beat eggs in large bowl. Remove cinnamon stick, if using; discard. Stir chocolate mixture and remaining sugar into eggs. Place cups in a 2-quart square baking dish on oven rack. Divide chocolate mixture among cups., Pour boiling water into baking dish around cups to depth of 1 inch.
Bake in a 325 degree F oven for 30 to 35 minutes or until a knife inserted near the center of each flan comes out clean. Remove cups or ramekins from water. Cool slightly on a wire rack. Cover and chill.
To unmold, loosen edges with knife, slipping point down sides of cups to let in air. Invert a dessert plate over each cup; turn cup and plate over together. Makes 4 servings.