1. Preheat your oven to 350°F (175°C).
2. Cut day-old bread into 1-inch cubes or toast fresh bread in the oven for 10 minutes to dry it out.
3. In a large mixing bowl, whisk together eggs, whole milk, heavy cream, sugar, vanilla extract, ground cinnamon, ground nutmeg, and salt to create the custard mixture.
4. Grease a 9×13 inch baking dish with butter or cooking spray.
5. Evenly distribute the bread cubes in the prepared baking dish. If using raisins, sprinkle them over the bread.
6. Drizzle melted butter over the bread cubes to lightly coat them.
7. Pour the custard mixture over the bread cubes, ensuring every piece is saturated. Gently press down the bread to help it absorb the liquid. Let sit for 15 minutes for optimal soaking.
8. Cover the baking dish with aluminum foil and bake for 30 minutes in the preheated oven.
9. Remove the foil and continue baking for an additional 25-30 minutes, or until the top is golden brown and a knife inserted into the center comes out clean.
10. Prepare the vanilla sauce by whisking heavy cream, whole milk, and sugar in a medium saucepan over medium heat until the sugar dissolves and the mixture steams (avoid boiling).
11. In a small bowl, mix cornstarch with one tablespoon of the warm cream mixture to make a smooth slurry.
12. Slowly whisk the cornstarch slurry into the saucepan while stirring constantly to thicken the sauce. Continue cooking and stirring until the sauce can coat the back of a spoon (about 5-7 minutes).
13. Remove the saucepan from heat and stir in butter, vanilla extract (or vanilla bean paste), and a pinch of salt. Mix until smooth and glossy.
14. Serve the warm bread pudding straight from the baking dish, drizzling it generously with the warm vanilla sauce.
15. For individual servings, portion the pudding into bowls or ramekins and add additional toppings like whipped cream, vanilla ice cream, fresh berries, or toasted nuts, if desired.
16. Refrigerate any leftovers in an airtight container for up to 3 days, reheating individual portions in the microwave or larger portions in a 300°F (150°C) oven.