Preheat the oven to 350°F / 180°C / Gas Mark 4, no fan (if you have a convection oven reduce the temperature by about 20° or follow the manufacturer's instructions).
Add both types of sugar and room temperature butter into a mixing bowl. Beat for about 1-2 minutes until well-combined. You can use a hand mixer or a stand mixer fitted with a paddle-shaped attachment.
Add the egg and vanilla extract. Beat for 1-2 minutes or until well-combined (scrape the sides of the bowl with a spatula).
Sift the flour, baking soda, cinnamon, and salt into the bowl. Mix over low speed until half-combined (the flour shouldn’t be fully incorporated at this point).
Add the chopped apples (they should be cut into very small cubes!), roughly chopped chocolate (or chocolate chips), and oats to the bowl. Mix until all the ingredients are evenly distributed.
You can bake the cookies immediately or put the cookie batter in the fridge for 30 minutes (cookies will be a little thicker and more chewy in the middle).
Scoop the dough with a 3-tablespoons ice cream scoop and roll the dough into balls between your hands (every ball should weigh about 40g/1.4oz). Don’t flatten the balls. Don’t form large cookie balls – the cookies will come out too large and flat. Smaller cookies come out thicker. Place the cookie dough balls on a baking sheet lined with parchment paper about 2 inches (5 cm) apart.
Bake for about 14-15 minutes if you’re baking the cookies straight away or for 15-16 minutes if the cookie dough has been in the fridge for 30 minutes. Bake the cookies in batches. The baking time may slightly vary depending on the oven. The cookies should have golden edges but will be completely soft.
Take the baking tray out of the oven and let the cookies cool on the baking sheet (they will continue baking on the baking sheet).
Enjoy!