Ready the oven and tray: Preheat your oven to 180°C (160°C fan) or 350°F. Line a large baking tray with parchment paper. The paper should lie flat and crisp; if it curls, a dab of butter under each corner will hold it still.

Mash the bananas: In your large bowl, mash the bananas with a fork until they form a chunky purée. You want a few small lumps remaining for texture, but no large pieces. The mixture should look almost like a thick, pale custard.

Combine the wet ingredients: Add the melted butter, brown sugar, egg, and vanilla extract to the mashed bananas. Beat with your mixer or a wooden spoon until the mixture is smooth, glossy, and smells deeply of vanilla. It should be a uniform, light caramel colour.

Mix the dry ingredients: In a separate small bowl, stir together the rolled oats, plain flour, bicarbonate of soda, cinnamon, and salt. Make sure the bicarbonate is evenly distributed; you do not want a bitter pocket in a single cookie.

Bring it together: Pour the dry mixture into the wet mixture. Stir gently with a spatula or spoon until just combined. The dough will be quite soft, almost like a thick porridge, and will smell warmly of oats and spice. Fold in the chocolate chips or walnuts if using.

Scoop and shape: Using a tablespoon or a cookie scoop, drop rounded mounds of dough onto the prepared baking tray, spacing them about 5cm apart. They will spread slightly, so leave room for the edges to meet without touching. The tops should look slightly craggy but moist.

Bake until golden: Place the tray in the preheated oven and bake for 12–14 minutes. At 12 minutes, the edges will be set and a deep golden brown, while the centres will still look slightly soft and pale. The kitchen will fill with a sweet, toasty banana and oat aroma.

Cool on the tray: Remove the tray from the oven and let the cookies rest on the hot tray for 5 minutes. This is crucial; they will be too soft to move immediately. After 5 minutes, carefully transfer them to a wire rack to cool completely. The bottoms should be golden and firm, while the tops remain soft.