Melt butter in a small saucepan over medium heat and whisk in brown sugar and corn syrup until smooth.
Pour the sugar mixture into a 9x13 inch baking dish and spread evenly across the bottom.
Arrange bread slices in the dish in two rows, overlapping slightly to fit the entire loaf.
In a large mixing bowl, whisk together eggs, half-and-half, vanilla extract, salt, and Grand Marnier.
Pour the egg mixture evenly over the bread slices, ensuring every piece is coated.
Cover the dish tightly with plastic wrap and refrigerate for at least 8 hours or overnight.
Remove the dish from the refrigerator 30 minutes before baking and preheat oven to 350°F (175°C).
Bake uncovered for 35 to 45 minutes until the bread is puffed and the top is golden brown.
Let the dish rest for 5 minutes before serving to allow the caramel base to thicken slightly.