In a heavy medium saucepan, combine 1 cup of the sugar with the lemon juice. Stir with a wooden spoon over moderate heat until the sugar dissolves, then cook, stirring occasionally, until the sugar turns deep brown, about 10 minutes.
Pour the hot caramel into an 8-cup metal ring mold; holding the mold carefully by the rim, tilt the mold to coat the interior with the caramel. If needed, use a wooden spoon to push the caramel up the side.
In a heavy medium saucepan, combine the milk with the remaining 3/4 cup of sugar. Cook over moderate heat, stirring occasionally, until the sugar dissolves.
Add the chocolate, cover and remove the pan from the heat. Set aside until the chocolate is melted. Stir.
In a medium bowl, lightly beat the eggs. Gradually whisk in the hot milk, vanilla and cinnamon until thoroughly combined. Strain the custard into a large glass measure, then pour it into the ring mold. Cover the flan loosely with foil and set the mold in a baking dish or roasting pan.
Pour in enough hot water to reach halfway up the side of the mold.
Bake in the center of the oven for about 1 hour and 10 minutes, or until the flan is set but still jiggly in the center.
Remove the mold from the baking dish.
Let the flan cool to room temperature, then refrigerate overnight. Run a small sharp knife around the sides of the mold, cover with a large rimmed plate and invert.
Cut the flan into wedges and serve.
Make Ahead: The flan can be refrigerated for up to 2 days.
