Melt Chocolate and Butter: In a medium heatproof bowl set over a saucepan of barely simmering water (a bain-marie), melt the chopped dark chocolate and cubed butter together. Stir occasionally until smooth and glossy. Remove from the heat and set aside to cool for about 10 minutes. It should be warm, not hot.

Whip Eggs and Sugars: While the chocolate cools, combine the caster sugar, light brown sugar, and room temperature eggs in the bowl of a stand mixer fitted with the whisk attachment (or use a large bowl and an electric hand mixer). Whisk on medium-high speed for a full 5-7 minutes. The mixture will become pale, very thick, and roughly triple in volume. When you lift the whisk, the batter should fall off in a slow, thick ribbon. This step is vital for the crinkle top.

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

Fold in Dry Ingredients: In a separate bowl, whisk together the plain flour, sifted cocoa powder, baking powder, and fine sea salt. Add this to the wet ingredients in two batches, folding gently with a spatula until no dry streaks remain. The dough will be very soft and glossy, similar to a thick brownie batter.

Chill the Dough: Cover the bowl with cling film and refrigerate for at least 30 minutes. This step is non-negotiable! It allows the dough to firm up, making it scoopable and preventing the cookies from spreading too much in the oven.

Prepare for Baking: Preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper.

Scoop and Bake: Using a medium ice cream scoop or two tablespoons, portion out balls of the chilled dough (about 40g each). Place them on the prepared trays, leaving at least 5cm of space between each one to allow for spreading. I find that an ice cream scoop is the best tool for uniform cookies that bake evenly.

The Final Touch: Sprinkle each dough ball generously with flaky sea salt.

Bake to Perfection: Bake for 10-12 minutes. The cookies are done when the edges are set and the tops are covered in beautiful cracks. The centre will still look slightly soft and underdone—this is what keeps them fudgy.

Cool: 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.