Arrange the bread slices in a single layer in an 8- by 12-inch baking dish. In the medium bowl of an electric mixer beat together the eggs, the milk, the Grand Marnier, the granulated sugar, the vanilla, and the salt until well blended. Pour the mixture over the bread, turning to coat evenly. Refrigerate the mixture, covered, overnight. Heat the butter in a large skillet over medium heat, add the bread slices, and saute until golden, about 4 minutes on each side. Sprinkle with the confectioners' sugar.