Preheat oven to 350. Generously spray a 3 to 4 quart ovenproof casserole/serving dish with cooking spray.
Prepare muffins by cutting into coarse pieces, about 1 inch square. Set aside. In large bowl, whisk eggs, milk, vanilla, salt, sugar, butter, flour and baking powder. Add muffin chunks and diced apples and blend well but gently. Take care to not break apart the muffin chunks. Let stand for 10 minutes.
Spoon into casserole. Put apple slices decoratively on top.
Bake for 45 to 55 minutes or til muffin filling (which becomes a custard) just begins to brown and appears lightly set. Remove from oven and drizzle on warm caramel sauce. cut into squares and serve warm, chilled or at room temperature with cream or creme anglaise. Makes 6 to 8 servings.
You can also make this one to two days ahead and refrigerate til you're ready to bake it.