In a heavy sauce pan, melt butter with brown sugar and corn syrup over moderate heat, stirring until smooth and pour into 9x13 inch baking dish.
Cut bread into 20 1"" slices. Arrange bread slices in one layer in baking dish on top of the sugar mixture, squeezing them slightly to fit.
In a bowl whisk together eggs, half and half, vanilla, Grand Marnier, salt and orange zest until well combined. Pour evenly over bread.
Chill bread mixture, covered at least 8 hours and up to one day.
Preheat oven to 350 degrees and bring bread to room temperature. Bake uncovered in middle of oven until puffed and edges are pale golden, 35-40 minutes.
Sprinkle with cinnamon (my preference), or powdered sugar.