Cream Butter and Sugars: In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, light brown sugar, and granulated sugar together on medium speed for about 3-4 minutes. The mixture should become light, pale, and fluffy. This step incorporates air, which is crucial for the texture.

Add Wet Ingredients: Add the egg, egg yolk, and vanilla extract to the butter mixture. Beat again on medium speed until everything is well combined, scraping down the sides of the bowl once or twice to ensure it's all incorporated evenly.

Combine Dry Ingredients: In a separate medium bowl, whisk together the plain flour, cocoa powder, bicarbonate of soda, and salt. Whisking first prevents clumps of cocoa or soda in your final cookies.

Mix the Dough: Add the dry ingredients to the wet ingredients in two batches. Mix on low speed until just combined. It’s important not to overmix here; stop as soon as you no longer see streaks of flour. Overmixing can lead to tough cookies.

Fold in the Chocolate: Add the chopped dark chocolate to the bowl and use a spatula to gently fold it through the dough until it's evenly distributed.

Chill the Dough: Cover the bowl with cling film and place it in the refrigerator to chill for at least 30 minutes. An hour is even better if you have the time. I find this step is absolutely essential to prevent the cookies from spreading too much in the oven. The dough will be firm and easier to handle.

Preheat and Prepare: While the dough is chilling, preheat your oven to 180°C (160°C fan). Line two large baking trays with parchment paper.

Portion the Cookies: Once chilled, use a medium ice cream scoop or two tablespoons to portion out balls of dough, each about 40-50g. Arrange them on the prepared baking trays, leaving plenty of space between them as they will spread (I usually fit 6-8 per tray).

Bake to Perfection: Bake one tray at a time for 12-14 minutes. The cookies are ready when the edges are set and the centres look slightly underdone. They will continue to cook on the hot tray.

Cool and Enjoy: Let the cookies cool on the baking tray for 5-10 minutes before carefully transferring them to a wire rack to cool completely. They will be very soft when they first come out of the oven. This waiting period allows them to set up perfectly.