Spread bread cubes on a large baking sheet and let dry overnight, or toast in a 250°F oven for 15 minutes, stirring halfway through. Transfer dried bread to a greased 9x13-inch baking dish.

In a large mixing bowl, whisk together milk, heavy cream, beaten eggs, sugar, bourbon, vanilla extract, cinnamon, nutmeg, and salt until smooth and well combined.

Stir 1/2 cup of the bourbon caramel sauce into the custard mixture until fully incorporated. Pour the custard evenly over the bread cubes, pressing down gently with a spatula to ensure all pieces are moistened.

Cover the dish with plastic wrap and refrigerate for at least 30 minutes or up to overnight, allowing the bread to absorb the custard completely.

When ready to bake, preheat your oven to 350°F (175°C). Remove the plastic wrap and drizzle melted butter over the top of the pudding.

Bake uncovered for 45-55 minutes, until the top is golden brown and crispy, and a knife inserted in the center comes out with just moist crumbs. The center should have a slight jiggle but be mostly set.

Remove from the oven and let rest for 10 minutes. Warm the remaining 1/2 cup bourbon caramel sauce and drizzle generously over the top before serving.

Slice into squares and serve warm with vanilla ice cream or whipped cream if desired.