Preheat oven to 325° F. Coat six 5-ounce ramekins with the butter. Place the bread in a large bowl. In a small saucepan, whisk together the cream, milk, and vanilla and warm over medium-high heat, stirring occasionally, just until the mixture comes to a boil. Remove from heat and add the chocolate chips, whisking until smooth. In a small bowl, whisk together the egg yolks and the sugar. Slowly whisk in a small amount of the hot chocolate cream, then whisk the egg mixture into the cream until thoroughly combined. Pour the mixture over the bread, stir, and press firmly with a spatula, making sure all the bread is soaked. Cover with plastic wrap and let sit for 20 minutes at room temperature. (It can soak in the refrigerator for up to a day.) Bring a large kettle of water to a boil. Remove the plastic wrap and divide the chocolate bread into the prepared ramekins. Press down gently and ladle any remaining liquid evenly among the ramekins, then place them in a large roasting pan. Transfer the pan to the oven and fill with boiling water until it reaches halfway up the sides of the ramekins. Bake until the pudding is set, about 25 minutes. Remove from oven and let cool at least 5 minutes in the water. Dust each ramekin with the cocoa. Serve warm or at room temperature.
This recipe can also be made using an 8-by-8-inch baking dish. Increase the cooking time to 40 minutes or bake until the pudding is set.