Grease or butter a 9x13 baking dish.
Slice the bread into 1-inch thick slices. Arrange bread in the baking dish, packing in tightly.
In a large bowl, whisk together the milk, half and half, eggs, cinnamon and vanilla. Pour over the top of the bread. Cover and refrigerate for 2 hours, or overnight.
When ready to bake, preheat oven to 350 degrees.
While oven is preheating, combine butter and brown sugar in a small saucepan over medium heat. Bring to a boil and stir until brown sugar has dissolved. Remove from heat and mix in the bourbon.
Pour over the top of the casserole, then bake in the oven for 35-40 minutes.