Preheat the oven to 350 degrees, and line an 8×8 inch baking dish with parchment paper. Spray the parchment with non-stick spray.
To make the crust and topping: In a large mixing bowl, combine the oat flour, whole oats, coconut sugar, peanut butter powder, cinnamon, baking soda, salt, and vanilla extract. Use a wooden spoon to stir all of this together, then add the coconut oil. Use the spoon or your hands to mix in the coconut oil until a crumbly dough starts to form. It should clump together a bit.
Measure out 1 cup of the crust mixture and set it aside in the fridge, we will use this for the topping. Press the remaining crust mixture into the bottom of the parchment lined baking dish, using your hands to press it down into an even layer.
Make the apple filling: In a medium sized saucepan, combine the chopped apples, maple syrup, arrowroot starch, vanilla, cinnamon and salt. Use a wooden spoon to stir it together to coat all of the apples in the arrowroot starch. Turn the heat to medium and bring the mixture to a simmer. Once simmering, turn the heat down to low and let simmer for another 5-10 minutes. Make sure you stir frequently to prevent the apples from burning. They will soften a bit and develop a thick sauce.
Remove from the heat and pour the mixture over the bottom layer of crust. Use the spoon to spread it out into an even layer.
Take the remaining crumble that you put in the fridge and evenly spread it over the top of the apple layer.
Bake for 30 minutes, until the middle layer is bubbling and the top is golden brown.
Allow the bars to cool for at least 30 minutes before removing them from the pan. Once cooled, transfer them to a wire rack to let them cool completely before cutting them. Store in the fridge to keep them fresh.