Preheat your oven to 350°F (175°C). Grease a baking dish (9x13 inches) with butter.
In a large bowl, combine the cubed brioche (or challah) with the whiskey. Toss to coat and let it soak for about 15 minutes, stirring occasionally.
In another mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, brown sugar, vanilla extract, ground cinnamon, nutmeg, and salt until well combined.
Pour the custard mixture over the soaked bread cubes. Gently fold the bread mixture with a spatula until all of the bread is coated and the liquid is absorbed.
Transfer the mixture to the greased baking dish. Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and a toothpick inserted comes out clean.
While the bread pudding is baking, prepare the caramel. In a saucepan over medium heat, combine the granulated sugar and water. Stir gently until the sugar dissolves, then stop stirring and let it boil until it turns a golden amber color.
Remove from heat, being cautious, and slowly add the heavy cream (it may bubble). Stir until combined, then add the whiskey and butter, stirring until smooth. Add a pinch of sea salt to enhance the flavor.
Once the bread pudding is done baking, remove it from the oven and allow it to cool slightly before slicing. Drizzle each serving with the whiskey caramel sauce.
Notes
For a non-alcoholic version, substitute whiskey with apple cider.