Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Mash the banana thoroughly in a medium mixing bowl using a fork until smooth. Add the peanut butter to the mashed banana and stir vigorously until well combined. Crack the egg into the mixture and stir until fully incorporated. Fold in the oats using a spoon. Add cinnamon and vanilla extract if using. Let the mixture rest for 2-3 minutes to allow oats to absorb moisture. Scoop the dough onto prepared baking sheet using a teaspoon, about 1 tablespoon portions. Flatten each cookie gently with the back of a spoon to about ¼-inch thickness. Bake for 12-15 minutes until edges are lightly golden but centers remain soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.