Preheat the oven to 375 F, and butter a 9×13 baking dish.
Combine the brown sugar, baking powder, salt, and flour in a bowl. Mix until incorporated, then cut in the butter and egg with a pastry cutter or forks. Mixture should be crumbly.
Pat ⅔ of the mixture into the prepared pan. With the remaining ⅓, add the oats and mix to combine. Set aside.
In a separate bowl, combine the sliced apples, sugar, corn starch, and cinnamon. Mix well.
Spread the peanut butter evenly over the crust, then top with the apple mixture and oat crumble mixture.
Bake for 40-45 minutes, or until the top is browning and the sides are bubbling with apple juice.
Remove and let cool before cutting and serving.
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/apple-peanut-butter-oatmeal-bars/