Mash the banana in a medium mixing bowl using a fork until smooth.
Stir in the ricotta, vanilla extract, chia seeds, and cinnamon (or cocoa powder). Let the mixture rest for 5 minutes to allow the chia seeds to thicken slightly.
Add the oats and mix well. For a sweeter version, stir in the date or maple syrup at this stage.
Gently fold in the chopped chestnuts until evenly distributed.
Preheat your oven to 180°C (350°F) and line a baking tray with parchment paper.
Air fryer: preheat it to 180°C (350°F) and lightly grease the basket if needed.
Scoop small portions of the dough (about golf ball-sized) and flatten them gently between your palms into cookie shapes.
Bake in the oven for 15 minutes, or until the cookies are golden brown.
In the Air fryer cook them for 10–12 minutes, checking for a golden finish.
Let the cookies cool on a wire rack before serving. Enjoy them warm or at room temperature.