1. Heat oven to 325 degrees. Lightly whisk the eggs and sugar together in a small bowl. Combine the milk and chopped chocolate in medium-size saucepan. Heat over medium-high heat until chocolate melts and mixture is smooth, about 5 to 7 minutes. Pour a small amount of milk mixture into egg mixture, whisking constantly. Whisk eggs back into milk mixture. Stir until sugar is dissolved.
2. Stir in vanilla and coffee liqueur. Pour through a strainer into a large glass measuring cup. Divide equally among 6 six-ounce custard cups. Transfer cups to a 13 x 9 x 2-inch baking pan; fill pan halfway with hot water. (You should need about 4 cups water.) Sprinkle custards with ground cinnamon.
3. Bake in 325 degree oven 25 minutes, just until set. Remove custard cups from water bath and cool on wire rack for 10 minutes. Serve warm with whipped cream, if desired. Makes 6 custards.