Grease a deep dish pie pan and add bread in a single layer.
In a small bowl add milk, eggs, vanilla extract, brown sugar, and cinnamon together. Mix well. 2 cups milk, 3 eggs, 3 tsp vanilla extract, 1/2 cup brown sugar, 1/2 tsp cinnamon
Pour the egg mixture over the bread–make sure it's poured evenly over the bread.
Prepare your cinnamon topping: Mix the brown sugar, flour, and cinnamon in a bowl. 1/4 c brown sugar, 1/4 c flour, 1/2 tsp cinnamon
Slowly mix in the butter with your fingers, breaking it up and mixing it all together. Don't stress if it's not all melted together–it'll be perfect after you bake it. 1 1/2 tbsp butter
Sprinkle the cinnamon topping over the uncooked casserole. Cover the casserole with plastic wrap and place in the fridge for a minimum of an hour or as long as overnight, or cook immediately.
Preheat oven to 350 degrees. Assuming your pan is in the fridge, pull the pan out of the fridge and let warm for 30 minutes on the counter (so you don't break your pan by putting it into a hot oven).
Bake foil-covered casserole for 45 minutes, then remove cover for last 15 minutes. Once it's golden brown, poke a knife in the middle to make sure it's cooked–it shouldn't feel or look too wet. Your cooking time could vary depending on the depth of your baking dish.