Slice the bread into roughly 1 inch cubes. Set aside.
Preheat the oven to 350 degrees.
Mist a 9 inch baking dish with cooking spray and set aside.
Stir the brown sugar and butter together in a small saucepan. Cook over medium heat, stirring occasionally, until the sugar and butter have melted and combined. This should take no longer than 2 minutes.
Remove the pan from the heat and spread into the bottom of your prepared 9 inch dish.
Add the bread pieces to the top of the brown sugar butter mixture.
In a separate bowl, or measuring cup, whisk together the milk, eggs, vanilla, and cinnamon.
Pour over the top of the bread, making sure to coat all of the pieces.
Bake for 1 hour, or until the top is lightly golden and a knife pulls out clean from the center of the pan.