Bring water to a boil in a saucepan. Split the vanilla bean and add to the boiling water. Add sugar, cinnamon, espresso, and a pinch of salt. Simmer for 1 minute and add the bittersweet chocolate. Whisk until it gets thick. Add milk and simmer 1 minute. Serve immediately with a dollop of vanilla ice cream and a sprinkling of chili powder on top.
You may make this ahead and reheat. Make sure to either whisk it or blend it in a blender to make it light and foamy before serving. (The classic Mexican chocolate was whisked with a special wooden tool called a 'molinillo'.)