Preheat and Prep: First, preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper. This is an important first step as the dough comes together quickly and should be baked immediately.

Melt the Chocolate: In a heatproof bowl set over a small pan of simmering water, gently melt the chopped dark chocolate and cubed butter. Stir occasionally until smooth and fully combined. Remove from the heat and set aside to cool for 5-7 minutes. It should be lukewarm, not hot.

Whip the Eggs and Sugar: In a separate large bowl, using a stand mixer or an electric hand mixer, whisk the eggs, caster sugar, and light brown sugar together on high speed. Continue whisking for a good 3-4 minutes until the mixture is very pale, thick, and has almost tripled in volume. When you lift the whisk, a ribbon trail should remain on the surface for a few seconds. This step is crucial for the crinkly top.

Combine Wet Ingredients: Pour the slightly cooled chocolate and butter mixture into the whipped eggs, along with the vanilla extract. Gently fold everything together with a spatula until just combined. Be careful not to knock out all the air you've just incorporated.

Sift the Dry Ingredients: In another bowl, sift together the plain flour, cocoa powder, baking powder, and fine sea salt. Sifting prevents lumps and helps create a finer crumb.

Fold it All Together: Add the dry ingredients to the wet mixture in two additions. Fold gently with your spatula until you no longer see streaks of flour. What works best for me is to stop mixing as soon as everything is incorporated to avoid developing the gluten, which keeps the cookies fudgy rather than cakey.

Add Chocolate Chips: If you're using them, now is the time to fold in your chocolate chips or chopped nuts.

Scoop and Bake: Using a medium ice cream scoop or two tablespoons, drop rounded heaps of the batter onto your prepared baking trays. Leave at least 5cm of space between each cookie as they will spread.

Bake to Perfection: Bake in the preheated oven for 10-12 minutes. The edges should be set and the tops will have that signature crinkle. The centres will still look a little soft and underdone – this is exactly what you want for a fudgy texture.

Cool Down: Let the cookies cool on the baking tray for 5 minutes before carefully transferring them to a wire rack to cool completely. They will be delicate when warm but will firm up as they cool.