About 3 tbsp unsalted butter
5 to 6 1/2 inch thick slices brioche
3 egg yolks
2 cups heavy cream
1 3/4 cups milk
2/3 cup light brown sugar
2 tsp vanilla extract
3/4 tsp cinnamon
1/2 tsp freshly grated nutmeg
1/2 tsp kosher salt
4 oz bittersweet chocolate, chopped
1 tbsp raw sugar (optional)
Heat oven to 325ºF. Place large roasting pan in oven, and bring a kettle of water to a boil. Generously butter brioche slices on one side; cut in half diagonally. In a large bowl, whisk together the eggs, egg yolks, heavy cream, milk, brown sugar, vanilla, cinnamon, nutmeg and salt. Sprinkle chocolate in a large casserole at least 3 inches deep.
Lay slices of bread, butter side up, over chocolate, overlapping slices like fish scales. Use just enough bread to cover all the chocolate. Pour custard on top, making sure to soak all the bread.
When kettle comes to a boil, place casserole inside roasting pan in oven and pour enough boiling water into roasting pan to come halfway up sides of casserole. Pudding is done when bread in center rises up and is bouncy to touch. Remove from oven. If you have a bitchen blowtorch, sprinkle sugar over top of pudding, and use torch to lightly caramelize sugar. If you do not have a blowtorch, skip this step (a broiler will curdle custard)
Pudding should be served warm, not hot.