Cut the bread into chunks and set aside.
In a large bowl, whisk together milk, eggs, brown sugar, and vanilla extract.
Place the pieces of bread in the bowl and press them into the egg mixture. Store mixture in the refrigerator overnight.
In a small bowl, combine flour, brown sugar, cinnamon, salt, and butter. Using a pastry cutter or a fork, work the butter into the flour mixture until it forms a sandy consistency. Store the butter mixture in the refrigerator overnight.
In the morning, place the bread mixture into the greased bowl of a slow cooker. You can also use a slow cooker liner to make clean up easier.
Discard any egg mixture remaining in the bottom of the bowl.
If desired, add blueberries to the slow cooker. (This is the step you skip if you don’t want to have blueberry french toast casserole!)
Top the bread and blueberries with the topping butter mixture.
Cook on LOW for 4-5 hours or on HIGH for 2-3 hours.