Preheat oven to 325°F. Grease a 9- by- 12-inch baking dish with cooking spray; set aside.
Using a serrated knife, remove the crust of the challah by cutting off a thin layer around the entire loaf. Cut the remaining bread into 1-inch cubes. Place cubes of bread inside prepared baking dish. Scatter raisins across the bread cubes.
In a stand mixer with the whisk attachment, beat together eggs, brown sugar, and granulated sugar on medium-high until tripled in volume, about 5 minutes. Add Grand Marnier, vanilla extract, and orange zest; mix to combine.
In a separate container, stir together milk, buttermilk, and melted butter. Slowly stream into egg mixture, beating on low until well combined. Pour custard over the bread and raisins, gently pressing on the bread pieces with a spoon to help the custard soak into the bread. Allow to rest for 15 minutes.
Spray one side of a sheet of aluminum foil with cooking spray and cover the baking dish with the greased side of aluminum foil. Place in preheated oven for 1 hour. Remove aluminum foil and bake until the top has dried slightly and the center of the bread pudding is set, about 30 minutes.
