Preheat the oven to 350 degrees F (175 degrees C).
Heat milk and 1/2 cup butter in a saucepan over medium heat until butter is completely melted, 3 to 5 minutes.
Mix sugar, eggs, cinnamon, and salt together in a large mixing bowl with a whisk until well blended. Stir in milk mixture. Stir in bread cubes, raisins, and hazelnuts. Let sit for 5 minutes. Pour into an ungreased 9×13-inch baking dish.
Bake, uncovered, until a toothpick inserted into the center comes out clean, 40 to 45 minutes.
While pudding is baking, heat brown sugar, 1 cup butter, and bourbon whiskey in a pot over medium heat, stirring constantly, until boiling and sugar is dissolved, about 5 minutes. Set aside.