Grease a 9x13-inch baking dish with butter; set aside.
In a large bowl, whisk together the eggs, granulated sugar, and salt until lightened, about 30 seconds; set aside.
In a medium-sized saucepan, whisk the brown sugar with cocoa. Whisk in the milk and cream, and heat over medium flame until warm. Whisk in the bittersweet chocolate, espresso powder, and cinnamon until the chocolate is melted. Whisk in the chocolate mixture into the egg mixture until combined. Whisk in the vanilla extract.
Add the bread cubes to the chocolate mixture and gently toss. Allow the bread to soak for 30 to 45 minutes, occasionally pressing the bread gently.
Adjust the oven rack to middle position and heat the oven to 165°C (325°F).
Gently pour the bread mixture into the prepared baking dish. Sprinkle with chopped semisweet chocolate.
Bake in the oven using the bottom heat function for 45 to 55 minutes. Remove and cool for 30 minutes before serving.
Prepare the Whipped Cream. In a chilled bowl of the stand mixer fitted with the whip attachment, whip the cream, sugar, and vanilla on medium-high speed until soft peaks form. Serve with the bread pudding.