Start by preheating your oven to 350°F (175°C). While it heats up, line a baking sheet with parchment paper to keep the cookies from sticking. This step is key to even baking and a clean release.
In a medium-sized bowl, whisk together the flour, baking soda, salt, and cinnamon. This ensures that all dry ingredients are evenly distributed and prevents clumps later in the dough. Set this bowl aside for now.
In a large bowl, use an electric mixer or whisk to beat the softened butter, brown sugar, and granulated sugar until the mixture becomes light and fluffy. This step helps trap air, giving the cookies a soft texture. Then, mix in the mashed banana, egg, and vanilla until everything is smooth.
Now, gently add the dry mixture into the wet ingredients. Do this gradually, mixing just until everything is combined—don’t overmix or the cookies may turn out dense. Once you have a uniform dough, fold in the chocolate chips using a spatula.
Using a tablespoon or small cookie scoop, drop rounded portions of dough onto your prepared baking sheet. Leave about 2 inches of space between each cookie. Bake for 10 to 12 minutes, or until the edges are golden and the centers look just set.
Once done, remove the baking sheet from the oven and let the cookies cool for 5 minutes right on the tray. Then, transfer them to a wire rack to cool completely. This step helps them firm up while staying chewy inside.