Preheat oven to 350°F. Place 6 (6 oz.) custard cups or ramekins in a 13-by-9-inch baking pan.
In a medium saucepan, scald milk, cream and cinnamon stick over medium heat. Whisk in instant-coffee granules. In a medium bowl, combine eggs, yolks and sugar, whisking until bubbly, about 1 minute. Gradually whisk in hot cream mixture and vanilla. Ladle custard into cups, discarding cinnamon stick.
Pour hot water around custard cups to come halfway up sides. Bake until custard is set but jiggly in center, about 30 minutes. Let custards cool in water bath for 20 minutes, then carefully transfer them to a rack and let cool to room temperature, about 40 minutes. Refrigerate custards until cold, about 3 hours. Garnish with whipped cream and cinnamon, if desired.