1. In a small saucepan, melt butter over medium heat. Add brown sugar and corn syrup, stirring until sugar is dissolved and mixture is smooth. Pour mixture into a 9×13-inch baking dish.
2. Arrange the bread slices in a single layer on top of the butter and brown sugar mixture. You can cut the bread to fit the pan if necessary.
3. In a large bowl, whisk together the eggs, half-and-half, vanilla extract, Grand Marnier (if using), and salt until well blended.
4. Pour the egg mixture evenly over the bread slices. Gently press the bread down to absorb the custard.
5. Cover the baking dish with plastic wrap and refrigerate overnight, or for at least 8 hours.
6. Preheat the oven to 350°F (175°C). Remove the plastic wrap from the baking dish and let sit at room temperature while the oven preheats.
7. Bake the French toast in the preheated oven for 35 to 40 minutes, or until puffed and golden brown. Let sit for a few minutes before serving. For an extra touch, sprinkle granulated sugar on top and use a kitchen blowtorch to caramelize until golden and crispy.
Variations and tips
For a seasonal twist, add a few teaspoons of cinnamon and a pinch of nutmeg to the custard mixture. Apple or pear slices arranged between the bread slices can also provide a delicious fruity touch. For a less sweet version, reduce the amount of brown sugar and butter, or use wholemeal bread for a heartier texture.