Let bread loaves partially thaw, but not rise. Cut or break one loaf into small pieces. Place on the bottom of a greased 9x13 pan. Cut up remaining loaf.
Combine butter, sugar, cinnamon (if using), milk, and pudding mix in a saucepan over medium/low heat, stirring to combine. Heat until smooth and butter is melted (warm, but not hot)
Pour over bread dough in pan. Arrange remaining dough pieces over the top evenly. Cover with saran wrap sprayed with non stick spray. Let rise in fridge, or away from any direct heat, overnight (it may rise a bit over the top of the pan).
Bake at 350 for 25-35 minutes. If the top starts to get dark, place a piece of aluminum foil over (lightly, not tight) so the middle can fully cook, but the top won't burn. When cooked, invert the pan onto a serving platter or cookie sheet. The caramel will run down the sides, lift off the pan. Enjoy!