Set the oven rack to the center position and preheat to 300ºF (149ºC).
Cut the bread into ¾-inch cubes.
Spread the bread on a large sheet pan or two smaller pans. Bake for 25 minutes, stirring halfway through toasting. The bread should be lightly golden, dry and crisp.
Cool on the tray for 10 minutes.
Grease the inside of a 13x9-inch casserole dish with softened butter.
Transfer toasted bread to the casserole dish and arrange in an even layer.
In a medium-sized bowl whisk eggs, 3 tablespoons brown sugar, vanilla, 2 teaspoons cinnamon, nutmeg, and salt until well combined.
Whisk in milk and heavy cream.
Pour the custard mixture evenly over the bread. Press the bread into the liquid to submerge. Allow the bread to soak up the custard mixture at least 4 hours or up to 24 hours. Some liquid will still remain on the edges but the bread should feel slightly mushy.
In a small bowl combine 1 tablespoon brown sugar and ½ teaspoon cinnamon. Right before baking sprinkle evenly over the top of the casserole.
Set the oven rack to the center position and preheat to 350ºF (177ºC).
Place the casserole dish on a large sheet pan. Bake until the custard is set, puffed up, and reaches an internal temperature in the center of 175 to 190ºF (79 to 88ºC), about 50 to 60 minutes.
If the bread starts to brown too quickly, loosely cover the top with foil after about 45 minutes of baking.
Cool for 30 minutes before serving. The casserole will slightly decrease in volume as it cools.
Sprinkle with powdered sugar right before serving and add desired toppings.