Heat the sugar in a medium saucepan over medium heat until it begins to melt. Cook, swirling the pan but not stirring, until deep amber, about 10 minutes. Stir in the orange zest and cook 30 seconds. Pour into a 9-inch-round cake pan; tilt to coat. Let harden, about 10 minutes.
Preheat the oven to 350 degrees F. Dissolve the espresso powder in 1 tablespoon warm water in a large bowl. Whisk in the eggs. Add the sweetened condensed milk, half-and-half, vanilla, cinnamon and 1/8 teaspoon salt; whisk until smooth. Pour the mixture through a fine-mesh sieve into the cake pan.
Loosely cover the cake pan with foil (do not let the foil touch the custard). Line a roasting pan with a kitchen towel and set the cake pan on top. Transfer to the oven and fill the roasting pan with hot water, about halfway up the side of the cake pan. Bake until the flan is set but still wobbly in the center, 1 hour, 30 minutes to 1 hour, 45 minutes. Remove from the oven and let cool in the water bath, about 1 hour. Remove from the water, cover with plastic wrap and chill at least 8 hours.
Run a knife around the edge of the pan to loosen the flan; let sit 20 minutes. Invert the flan onto a rimmed plate and unmold, letting the caramel drip out. Cut into wedges.
Photograph by Con Poulos
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