Grease a 9” x 13” baking dish with butter (or nonstick cooking spray, if you must). Place baguette slices into baking dish.
In a medium bowl, whisk together the milk, eggs, vanilla and cinnamon. Once combined, pour this mixture evenly over the bread slices.
Cover baking dish with plastic wrap and refrigerate overnight.
Heat oven to 350°Uncover baking dish and bake for 30 to 35 minutes or until top is golden brown.
While French Toast is baking, make Sweet Topping: Melt the butter in a medium saucepan, then whisk in the brown sugar and maple syrup. Cook and whisk until well combined and slightly thickened.
Pour Sweet Topping evenly over the warm, baked French Toast. Serve immediately with fresh blueberries. Serves 4 or more.