Prepare two 9x13 baking dishes with nonstick cooking spray. Divide the cubed bread between the two dishes.
In a large bowl, whisk together the first 6 ingredients until well blended. Divide the mixture evenly between the two casseroles dishes and pour over the bread.
Drop berries over the top.
Cover with aluminum foil and refrigerate overnight or freeze up to 3 months.
When ready to bake, preheat the oven to 350 degrees.
While the oven preheats, remove the french toast casseroles from the refrigerator and allow them to rest on the counter.
Bake the casseroles, covered for 30 to 45 minutes or until a knife inserted comes out clean.
If topping, combine the butter with the brown sugar using a fork.
Remove the casseroles from the oven and top with the brown sugar mixture. Return to the oven and bake, uncovered, for 15 to 20 minutes longer.
Serve with syrup or whipped topping.