Caramel: Combine sugar and water in a saucepan. Cook over medium-high heat, stirring with a wooden spoon just until sugar dissolves. Then bring to a boil without stirring. Continue cooking to a medium to deep amber color, depending on desired flavor. The deeper the color, the more distinctive the flavor. Be careful not to burn the caramel. Immediately pour caramel into a 9-inch round cake pan, swirling to coat bottom. (Do not use a pan with removable bottom.) Place pan in a roasting pan; set aside.
Custard: Preheat oven to 350 degrees Fahrenheit. Heat milk in a saucepan to simmer. Add orange peel and cinnamon stick to milk. Cover and let stand 10 minutes. Return milk mixture to boil.
Lightly beat eggs and sugar with vanilla extract in bowl. Remove and discard orange peel and cinnamon from milk with slotted spoon. Gradually whisk hot milk into eggs. Pour custard into the cake pan over the caramel. Place pans in oven. Pour enough boiling water into roasting pan to come halfway up sides of cake pan.
Bake 45 to 50 minutes, until knife inserted 2 inches from edge comes out clean. Let stand in water bath 10 minutes. Cool. Cover and refrigerate up to 24 hours.
Cut around edge and invert onto a large serving plate.