Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper to prevent sticking, and set them aside for later use.

In a medium bowl, combine the whole wheat flour, rolled oats, baking soda, salt, and ground cinnamon. Whisk until everything is evenly mixed. Set this dry mixture aside.

In the bowl of an electric mixer, blend together the smooth peanut butter and packed dark brown sugar until the mixture is creamy. Add the egg and vanilla extract, and mix until thoroughly combined. Next, add the mashed banana and blend until just incorporated. I like to make sure the banana is mashed very well for even moisture throughout the cookies.

Gradually add the dry ingredient mixture from Step 2 into the wet ingredients from Step 3, mixing until just combined to avoid overworking the dough. Gently stir in the shredded apple and chocolate chips, if using. If you want your cookies extra fruity, I recommend using a slightly tart apple for a nice balance with the sweetness.

Scoop out 3-4 tablespoons of dough for each cookie, forming large balls with your hands. Arrange them a few inches apart on your prepared baking sheets. Flatten each dough ball gently using a cup measurer or glass sprayed with non-stick cooking spray. Bake the cookies in the preheated oven for 10-12 minutes until they are slightly browned.

Let the cookies cool on the baking sheets for about 10 minutes after baking, then transfer them to wire racks to cool completely before enjoying. I find that letting the cookies fully cool helps set their chewy texture.