Grease a 9x13 inch pan or something similar with nonstick spray. Cube the bread into 1 inch pieces and spread over 9x13 pan. It is important that bread is slightly old or otherwise it will be too soggy. Set aside.
Whisk eggnog, eggs, brown sugar, vanilla and spices in large bowl until no lumps remain. Pour over the bread and cover tightly with plastic wrap. Refrigerate for at least 3 hours, but overnight is best.
Preheat oven to 350°F and remove pan from refrigerator. Bake for 35-45 minutes or until golden brown on top.
Make the sauce by melting the sugar, eggnog and butter in a saucepan over medium heat. Do not bring to a bowl. Once dissolved, add the rum. Pour over each slice of bread pudding before serving.
Notes
Make sure your bread is stale and a little crunchy or you will have a soggy bread pudding.
You can reduce the sugar by ¼ cup if you want. I've made it with less and haven't noticed a difference.
You can half this recipe in an 8 or 9-inch baking pan. However the bake time will be shorter, around 25-30 minutes.
If you want some added crunch, add ½ cup of pecans or walnuts.