In a small heavy saucepan, melt butter with brown sugar and corn syrup over moderate heat, stirring, until smooth. Pour into a 9x13 inch baking dish.
Cut six 1" thick slices from center portion of bread, reserving ends for another use. Trim crusts, if desired. Arrange bread slices in one layer in baking dish, squeezing them to fit.
In a bowl, whisk together eggs, half-and-half, vanilla, Grand Marnier and salt until combined well; pour evenly over bread. Cover and refrigerate mixture at least 8 hours and up to 1 day.
Remove mixture from refrigerator 30 minutes prior to baking.
Preheat oven to 350 degrees.
Bake bread mixture, uncovered, in middle of oven until puffed and edges are pale golden, 35-40 minutes.