In a small heavy saucepan melt butter with brown sugar and corn
syrup over moderate heat, stirring, until smooth and 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. and bring bread 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.