Prepare the Caramel Base:

1. Melt the butter in a small saucepan over medium heat.
2. Stir in the brown sugar and corn syrup until smooth and fully dissolved.
3. Pour the caramel mixture evenly into a 9×13-inch baking dish, spreading it across the bottom.

2. Arrange the Bread:

1. Layer the bread slices over the caramel in a single layer, slightly overlapping if necessary.

3. Make the Custard:

1. In a large mixing bowl, whisk together eggs, half-and-half, vanilla extract, Grand Marnier (if using), and salt.
2. Pour the custard evenly over the bread, ensuring every piece is well-soaked.

4. Refrigerate Overnight:

1. Cover the dish with plastic wrap or foil.
2. Refrigerate for at least 8 hours or overnight to let the bread absorb the custard.

5. Bake the French Toast:

1. Preheat your oven to 350°F (175°C).
2. Remove the dish from the fridge and let it sit at room temperature for 20 minutes.
3. Bake uncovered for 40-45 minutes, or until puffed and golden brown on top.

6. Cool and Serve:

1. Allow the dish to cool for 5-10 minutes to let the caramel set slightly.
2. Serve warm with fresh berries, whipped cream, or a dusting of powdered sugar for an extra flourish.