Place the chopped cinnamon buns into a large bowl.
Whisk together the eggs, milk, vanilla, spice and sugar. Pour over the bread and stir until the bread is coated completely.
Pour into the 9x13 pan, spreading out evenly and pushing it down slightly to compress. Cover with foil and place in the fridge overnight.
In the morning, pre-heat your oven to 350 °F.
Place the pan in the oven and bake, covered, for 30 minutes. Remove the foil and continue baking for another 20-25 minutes until the center of the casserole is cooked and set and has reached a minimum temperature of 165 °F.