Line the bottom of a greased 13 x 9 x 2 inch baking dish with 6 slices of bread. Butter remaining slices; place with butter side up over bread in pan. In a mixing bowl, beat eggs. Add milk, cream, sugar and vanilla; mix well. Pour over bread; let stand for 15 minutes. Place the dish in a large baking pan. Pour boiling water into larger pan to a depth of 1 inch. Bake, uncovered, at 375oF for 40 minutes or until a knife inserted neat the center comes out clean. Let stand for 10 minutes before serving. Top with preserves and whipped cream if desired.