Preheat oven to 375 degrees F. Butter a 2-quart baking dish.
In a medium bowl, combine oats, brown sugar, baking powder, cinnamon and salt. Stir in half of the walnuts, raspberries and chocolate. Pour into the prepared baking dish.
Using the same bowl, whisk together the milk, egg, vanilla and melted butter. Pour over oats mixture and jiggle the baking dish a bit to make sure the liquid soaks the oats.
Sprinkle the remaining nuts, raspberries and chocolate over the top. Bake for about 35 minutes. If you like, you can sprinkle a little sugar over the top and put it back under the broiler for a minute or two to caramelize.