Arrange the bread slices in a lightly greased 3-quart baking dish. Overlap the slices if needed.
Combine the eggs, milk, brown sugar, cinnamon, and vanilla. Pour the mixture over the bread in the pan. Lightly press the bread into the egg mixture as needed to thoroughly moisten the bread.
Cover and refrigerate at least 8 hours or up to 24 hours.
To make the topping:
Preheat oven to 350°F (175°C).
Place the apple slices and pecans over the top of the chilled filling.
Mix the butter and brown sugar. Drizzle over the apples and pecans.
Bake 35 to 40 minutes, or until a knife inserted near the center comes out clean. Cool 10 minutes before serving.
If desired, serve with confectioners' sugar and maple syrup.