Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats and set aside.

In a medium bowl whisk together the all-purpose flour, rolled oats (if using), baking powder, baking soda, ground cinnamon and fine sea salt until evenly combined; set this dry mix aside.

In a large mixing bowl, cream the softened unsalted butter and packed brown sugar together using a handheld mixer or stand mixer fitted with the paddle attachment until the mixture is pale and smooth, about 2 to 3 minutes. Scrape down the bowl once during mixing.

Beat in the large egg and the pure vanilla extract until the mixture is uniform. Add the mashed ripe banana and mix just until combined — the batter will look loose compared with a regular cookie dough but should hold some shape.

Gradually fold the dry flour-oat mixture into the wet mixture using a rubber spatula or low mixer speed. Stir until there are no streaks of flour, but avoid overmixing which can toughen the cookies. If the dough is excessively wet and doesn’t hold any shape, add up to 1 tablespoon of milk or buttermilk to adjust.

Fold in any optional mix-ins such as chocolate chips or chopped walnuts until evenly distributed. The cookie dough should be scoopable with a cookie scoop or spoon.

Using a medium cookie scoop (about 1 1/2 tablespoons) or two spoons, drop rounded mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart. For a slightly domed look, mound the dough taller rather than flattening it. Optionally sprinkle a pinch of coarse sugar on top of each mound for sparkle and extra caramelization.

Bake in the preheated oven for 11 to 14 minutes, rotating the sheet halfway through baking for even color. The cookies should be golden brown on top with darker caramelized spots and still look slightly glossy and soft in the centers.

Remove the baking sheet from the oven and let the cookies rest on the sheet for 5 minutes to finish setting, then transfer them to a wire rack to cool completely.

Taste one once cooled and adjust for future batches — if you want a deeper banana flavor, use riper bananas or add a teaspoon of banana extract; if the cookies spread too much, reduce butter by 1 tablespoon or chill the dough for 30 minutes before baking.