In a small heavy saucepan melt butter with brown sugar and corn syrup over moderate heat, stirring, until smooth and pour into a 13x9x2 inch baking dish. Cut 6 (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 degrees 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.