In a large mixing bowl, whisk the eggs until well combined. Add the milk, heavy cream, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt. Whisk until everything is fully incorporated.

Gently fold in the cubed bread into the custard mixture until all pieces are coated. If using, mix in the raisins. Allow the mixture to rest for 20-30 minutes while the oven preheats to 350°F (177°C).

Coat the bottom and sides of a casserole dish with 1 tablespoon of melted butter. Pour the bread pudding mixture into the prepared dish.

Bake in the preheated oven for 50-60 minutes, or until the top is golden brown and no custard seeps through when gently pressed in the center.

In a small saucepan, combine the milk, heavy cream, granulated sugar, butter, and cornstarch. Heat over medium heat, stirring occasionally until the butter melts. Once melted, stir more frequently until the sauce thickens enough to coat the back of a spoon, about 10-12 minutes.

Remove the pan from heat, stir in the vanilla extract, and transfer the sauce to a heat-safe bowl to cool slightly. The sauce will continue to thicken as it cools.

Once the bread pudding is done baking, allow it to cool for 15-20 minutes. Serve warm, drizzling with vanilla sauce and reserving extra sauce for individual servings. Enjoy warm from the oven, at room temperature, or chilled.