In a small heavy saucepan melt butter with brown sugar and corn syrup over moderate heat, stirring, until smooth.
Pour into a 13- by 9- by 2-inch baking dish.
Cut six 1-inch thick slices from center portion of bread, reserving ends for another use, and trim crusts.
Arrange bread slices in one layer in baking dish, squeezing them slightly to fit.
In a bowl whisk together eggs, half-and-half, vanilla, Grand Marnier, and salt until combined well and pour evenly over bread.
Chill bread mixture, covered, at least 8 hours and up to 1 day.
Preheat oven to 350° F
While oven is preheating, allow bread to come to room temperature.
Bake bread mixture, uncovered, in middle of oven until puffed and edges are pale golden, 35 to 40 minutes.
Serve hot French toast immediately.