Spread softened butter on one side of each bread slice. Arrange 12 bread slices, buttered side down, slightly overlapping in a single layer in a 13 x 9-inch baking dish coated with cooking spray. Spread marmalade evenly over bread; top with remaining 12 bread slices, buttered side up.
Combine milk and next 4 ingredients (through eggs), stirring with a whisk. Pour egg mixture over bread. Cover and refrigerate 8 hours or overnight.
Preheat oven to 350°.
Sprinkle casserole with walnuts. Bake at 350° for 45 minutes or until golden. Let stand 5 minutes before serving.