Preheat oven to 350 degrees F. Add bread cubes to a large baking sheet in a single layer. Bake for 15 minutes, stirring halfway through, or until toasted; set aside.
Meanwhile, chop 12 ounces of chocolate. Add 8 ounces chopped chocolate to a medium mixing bowl, set remaining 4 ounces chocolate aside.
Microwave the heavy cream and milk (preferably in a glass liquid measuring cup) until simmering, but not boiling, about 2 minutes. Pour over 8 ounces chopped chocolate in the bowl and let stand for 3 minutes, then vigorously whisk until chocolate is completely smooth; set aside.
In a large mixing bowl, whisk together the eggs, egg yolks, sugar, cocoa powder, vanilla extract, cinnamon, salt and nutmeg. Slowly whisk in the chocolate mixture until smooth (this is now the chocolate custard).
Add half of the bread cubes, half of the remaining chopped chocolate (2 ounces), and half of the chocolate custard to a lightly greased 9x13 casserole dish. Stir until evenly coated. Add the remaining bread cubes and chopped chocolate (2 ounces) to the chocolate custard in the bowl. Stir until evenly combined then pour into the pan and spread into an even layer.
Cover the pan with foil and allow to sit at room temperature for 45-60 minutes, until the bread soaks up most of the custard.
Bake the covered bread pudding for 45-50 minutes, until the edges appear set and a toothpick inserted in the center comes out with few moist crumbs, but no raw/wet custard. The last 5 minutes of cooking, make the ganache.