Preheat oven to 375 degrees F (190 degrees C). Grease 6 cupcake cups, preferably in a dark-colored metal pan.
Place 1 chopped bar of bittersweet chocolate, unsweetened chocolate, and butter in a microwave-safe bowl; place bowl in microwave, and cook on low power until the butter has melted and the chocolate is soft, 2 to 3 minutes. Check and stir often to avoid burning the chocolate. Stir until smooth.
In a mixing bowl, whisk cinnamon, vanilla extract, eggs, egg yolk, sugar, and salt until thoroughly combined, and stir in the flour just until blended. Mix in the chocolate mixture, stir the batter a few times until smooth, and lightly mix in the 1/2 bar of bittersweet chocolate broken into 1/2-inch pieces. Spoon batter into the prepared cupcake cups, filling them about 3/4 full.
Bake in the preheated oven until a knife inserted into the center of a cake comes out with streaks of thick batter, about 18 minutes. The tops of the cakes should be almost firm. Allow to cool in the pan on a rack for 5 to 10 minutes to serve warm, or 20 minutes to serve at room temperature.
