Preheat the oven to 325 degrees F. Butter a 2-quart baking dish and set aside. Whisk the yolks, whole eggs and sugar together in a large bowl. Bring the milk, vanilla, cinnamon, nutmeg and salt to a boil in a medium saucepan. Whisking constantly, slowly pour the milk mixture into the egg mixture.
Toss the bread and chocolate together in a large bowl and then transfer to the prepared baking dish. Pour the egg-milk mixture over the bread. Stir the bread a few times so the custard settles evenly in the pan and set aside for 10 minutes. Scatter the butter pieces over the bread, cover tightly with foil and put the dish inside a larger baking dish or roasting pan. Pour enough hot tap water into the larger dish to reach halfway up the side of the bread pudding dish. Bake until just set, but still wiggly, about 50 minutes.
Uncover and cool completely. Serve with a dollop of whipped cream and a sprinkling of chocolate shavings if using.
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