Heat oven to 350°F. Grease a 9x13-inch baking dish with a little butter.
Cube the bread and spread it in the dish. Sprinkle raisins evenly over the top.
Melt the butter gently and let it cool for a minute.
Whisk eggnog, beaten eggs, sugar, vanilla, nutmeg, cinnamon, and salt until smooth and well combined.
Whisk in the melted butter, then stir in bourbon or rum if using.
Pour the custard evenly over the bread. Press gently so all cubes soak. Let stand 10–12 minutes.
Set the dish on a rimmed baking sheet. Bake 40–45 minutes until puffed, golden, and the center barely jiggles.
Rest 10 minutes. Dust with powdered sugar and serve warm with whipped cream.