Preheat to 350°F (175°C). Set an 8-inch (20 cm) round cake pan inside a 13×9-inch pan and pour hot water into the outer pan to ½ inch (1.25 cm) depth.
In a small saucepan over medium heat, melt the sugar, stirring until amber. Immediately pour into the 8-inch pan and, using oven mitts, swirl to coat bottom and sides. Return the pan to the water bath.
Combine chocolate and evaporated milk in a microwave-safe bowl. Heat in 30-second intervals, stirring, until smooth.
In a blender, blend the chocolate-milk mixture, sweetened condensed milk, eggs, cinnamon, and vanilla until silky.
Pour into the caramel-lined pan. Bake for about 50 minutes, until a thin knife near the center comes out mostly clean with a slight jiggle.
Cool on a rack 30 minutes, then refrigerate 4 hours or overnight until fully set.
Run a knife around the edge, invert a plate over the pan, and flip. Tap or gently shake to release. Keep chilled until serving.