Grease a 9- by 13-inch baking dish with butter; set aside. In a large bowl, whisk eggs, granulated sugar, and salt to combine; set aside.

In a medium saucepan, whisk brown sugar and cocoa powder to combine. Whisk in milk, cream, and vanilla extract, and heat over medium heat until warm. Add bittersweet chocolate, espresso powder, and ground cinnamon, whisking well until chocolate is melted, 2 to 4 minutes. Remove from heat and let cool slightly, about 3 minutes. Add chocolate mixture to egg mixture, whisking well to combine.

Add bread to 9- by 13- inch baking dish. Pour chocolate mixture over and gently toss to evenly coat bread. Let sit until bread is thoroughly saturated, 30 to 45 minutes, stirring occasionally with a flexible spatula to ensure bread is thoroughly saturated. Adjust oven rack to middle position and preheat to 325ºF (165ºC).

Tuck chopped bittersweet chocolate into bread pudding, evenly distributing it. Bake until pudding is just set in the center, 45 to 55 minutes. Remove from oven and let cool about 30 minutes before serving.

For the Whipped Cream: In the bowl of a stand mixer fitted with the whisk attachment, whip heavy cream, sugar, and vanilla on medium-high speed until medium peaks form, about 3 minutes. (Alternatively, in a medium bowl, combine heavy cream, sugar, and vanilla. Using an electric mixer fitted with the whisk attachment, beat on medium-high speed until medium peaks form, about 3 minutes.) Serve with bread pudding.