Cream Butter and Sugars: In a large bowl using a stand mixer or an electric hand mixer, beat the softened butter with the soft light brown sugar and caster sugar on medium speed for about 3-4 minutes. You're looking for a light, fluffy, and pale mixture.

Add Wet Ingredients: Add the egg and vanilla extract to the creamed butter and sugar. Beat again for another minute until everything is well combined. Scrape down the sides of the bowl to ensure it's all incorporated evenly.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, ground cinnamon, and salt. This ensures the raising agent and seasoning are distributed evenly.

Mix the Dough: Gradually add the dry ingredients to the wet ingredients, mixing on a low speed until just combined. Be careful not to overmix at this stage; stop as soon as you no longer see streaks of flour.

Fold in Oats and Chocolate: Gently fold in the rolled oats and dark chocolate chips using a spatula until they are evenly distributed throughout the dough. The dough will be quite thick and sticky.

Chill the Dough (The Important Step!): Cover the bowl with cling film and place it in the refrigerator to chill for at least 1 hour. This solidifies the butter, which prevents the cookies from spreading too much, and allows the flavours to deepen. You can chill it for up to 48 hours.

Preheat and Prepare: When you're ready to bake, preheat your oven to 180°C (160°C fan). Line two large baking trays with parchment paper.

Shape the Cookies: Scoop out rounded tablespoons of the chilled dough (about 40g each). I find that using a medium ice cream scoop gives you perfectly uniform cookies. Roll them into balls and place them on the prepared baking trays, leaving at least 5cm of space between each one to allow for spreading.

Bake to Perfection: Bake for 12-15 minutes, or until the edges are golden brown but the centres still look a little soft and slightly underdone. They will continue to cook on the hot tray.

Cool Down: Let the cookies cool on the baking tray for 5 minutes before transferring them to a wire rack to cool completely. This helps them set properly.