In a small saucepan, melt butter with brown sugar and corn syrup over moderate heat, stirring until smooth, and pour into a 13" x 9" x 2" Pyrex baking dish.
Cut 1-inch thick slices from bread, reserving ends for another use.
Arrange bread slices in one layer in baking dish, squeezing them to fit.
Cut smaller pieces of bread to fill in gaps.
In a bowl, whisk together eggs, half and half, vanilla, Grand Marnier, and salt until well combined; pour evenly over bread.
Chill bread mixture, covered, at least 8 hours (overnight) and up to 1 day.
Preheat oven to 350 degrees and bring bread to room temperature.
Bake bread mixture, uncovered, in middle of oven until puffed and edges are pale golden, 35-40 minutes.
Remove from oven and let sit about 5 minutes. Run a knife around the edge to loosen.
