Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy.
Add the mashed banana, egg, and vanilla. Stir until combined—it’ll look a bit lumpy, don’t worry.
In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, and nutmeg.
Slowly add the dry ingredients to the wet mixture, stirring just until combined. Don’t overmix.
Fold in the walnuts or chocolate chips if using.
Drop spoonfuls of dough onto the baking sheet and gently flatten with the back of a spoon.
Bake for 10–12 minutes, or until the edges are lightly golden.
Cool on the pan for 5 minutes, then transfer to a wire rack to cool completely—if they make it that far.