1. Preheat oven to 350 degrees F. If your bread isn’t dry enough, divide bread cubes evenly in a single layer among two baking sheets. Toast, stirring halfway through, until bread is hard and dry but still pale, about 5-10 minutes.*
2. Grease a 9/x13-inch baking dish with butter. Add bread cubes and gently shake the dish so the cubes settle evenly. If you’re adding pecans, sprinkle them over top and gently stir or shake so some of the pecans fall into the middle.
3. In a large bowl, whisk together eggs until beaten. Add sugar, vanilla, cinnamon, nutmeg, salt, and eggnog, and whisk until combined.
4. Slowly and evenly pour eggnog mixture over the top of the bread cubes. The mixture should come to just below the top of the bread, with edges and corners poking out the top. If you want the top of your  bread pudding to be flat, press the bread down into liquid until every piece is submerged.
5. Let bread sit for 20-30 minutes so it can soak up the liquid (you can also let sit covered in the refrigerator overnight).*
6. Bake until the pudding is set, the center puffs up and jiggles slightly, and a thermometer inserted into the center reads at least 160 degrees F, about 45-55 minutes.
7. Let dish cool on a wire rack for at least 25 minutes before serving.
8. Optional: Garnish with powdered sugar and drizzle butter rum sauce over top.

For the hot buttered rum sauce (optional):

1. In a small saucepan over medium heat, add butter, sugar, brown sugar, heavy cream, and dark rum and stir until melted. Let simmer until slightly thickened, about 2-3 minutes, and remove from heat.
2. Let cool in the pan for at least 30 minutes (sauce will continue to thicken as it cools).