Melt butter and brown sugar together in a saucepan over medium-low heat (2–4 min). Pour into a greased baking dish.
Cube bread and scatter bread pieces over the top in a 1½–2 inch layer.
Whisk eggs, milk, and vanilla. Pour over bread and gently press down so it soaks it all up. Cover and refrigerate 8 hours or overnight.
In the morning, remove from fridge, preheat oven to 350°, and bake uncovered for 50–60 minutes until golden and set.
