Heat brown sugar and corn syrup in a heavy saucepan over moderate heat stirring constantly until smooth. Do not boil.
Pour evenly over bottom of baking dish.
Slice the bread, crust can be trimmed off, do not use end pieces.
Arrange to cover sugar mixture in single layer slightly squeezing pieces to cover bottom of dish.
Whisk together the eggs, half-and-half, vanilla, Gran Marnier and salt until well blended.
Pour evenly over bread. Cover with plastic wrap and chill for at least 8 hours but not more than 24.
Set casserole out to come up to room temp. and preheat oven to 350.
Bake uncovered in middle of oven until edges are
puffy and a pale golden brown. 35 ~ 40 minutes.