Preheat and Prep: Preheat your oven to 180°C (160°C fan). Line two large baking trays with parchment paper.

Melt the Chocolate and Butter: Place the 180g of chopped dark chocolate and the cubed butter in a heatproof bowl. Set it over a saucepan of gently simmering water (a bain-marie), making sure the bottom of the bowl doesn’t touch the water. Stir occasionally until everything is completely melted and glossy. Remove from the heat and set aside to cool slightly for about 5 minutes.

Whip the Eggs and Sugar: In a separate large bowl, using a stand mixer with the whisk attachment or a hand mixer, beat the eggs, caster sugar, and light brown sugar on high speed. Continue whipping for 5-7 minutes, until the mixture is very pale, thick, and has tripled in volume. When you lift the whisk, it should leave a ribbon trail that sits on the surface for a few seconds. This step is crucial for the crackly top.

Combine Wet Ingredients: Pour the slightly cooled chocolate-butter mixture into the whipped eggs and sugar. Add the vanilla extract. Gently fold everything together with a spatula until just combined. Be careful not to knock all the air out of the eggs.

Fold in Dry Ingredients: Sift the plain flour, cocoa powder, baking powder, and salt directly over the wet mixture. Fold gently with the spatula until there are no dry streaks left. The batter will be thick and mousse-like. Finally, fold in the 100g of chocolate chips.

Rest the Dough: This is an important step! Let the cookie dough sit at room temperature for 10 minutes. It will thicken up slightly, which helps control spreading in the oven. What works best for me is using this time to clear away my prep bowls.

Scoop and Bake: Drop rounded tablespoons of the dough onto your prepared baking trays, leaving at least 5cm (2 inches) of space between each one as they will spread. Sprinkle each mound of dough with a little flaky sea salt.

Bake to Perfection: Bake for 10-12 minutes. The cookies will puff up and the tops will look crackly and set, but the centres will still be very soft. They might look underdone, but they will continue to firm up as they cool. This is the secret to a fudgy centre.

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