Prepare Your Workspace: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Line two large baking trays with parchment paper. This ensures your gooey flourless chocolate fudge cookies won't stick and makes for easy cleanup.

Melt Chocolate and Butter: In a heatproof bowl set over a pan of simmering water (bain-marie) or in a microwave-safe bowl, combine the chopped dark chocolate and unsalted butter. Stir occasionally until the mixture is completely smooth and glossy, with a rich, dark brown sheen. Remove from heat and allow to cool slightly.

Combine Wet Ingredients: In a separate large bowl, whisk together the caster sugar, eggs, and vanilla extract until pale yellow and frothy. The mixture should feel lighter and slightly aerated when you whisk it vigorously.

Integrate Chocolate Mixture: Pour the slightly cooled melted chocolate and butter mixture into the egg and sugar mixture. Fold gently with a spatula until just combined, creating a thick, dark, and wonderfully aromatic base that smells intensely of chocolate.

Add Dry Ingredients: Sift the cocoa powder, baking powder, and salt directly into the wet ingredients. Fold carefully until no streaks of dry ingredients remain. The batter will be thick, dark, and have a beautiful, uniform consistency. Finally, stir in the chocolate chips, distributing them evenly throughout the rich chocolate cookies dough.

Chill the Dough: This step is crucial for achieving that perfect texture. Cover the bowl with cling film and refrigerate the dough for at least 30 minutes, or up to 2 hours. The dough will firm up, making it easier to handle and preventing excessive spreading in the oven.

Portion and Bake: Using a spoon or a small ice cream scoop, drop rounded tablespoons of the chilled dough onto the prepared baking trays, leaving ample space (about 5-6 cm) between each cookie. These chewy chocolate cookies will spread. If desired, sprinkle a pinch of flaky sea salt on top of each cookie. Bake for 10-12 minutes. The cookies will look slightly puffed and set around the edges but will still appear very soft and gooey in the centre when you gently press one.

Cooling: Remove the trays from the oven. The cookies will still be very soft, so let them cool on the baking trays for 5-10 minutes. This allows them to set up further and prevents them from breaking. Once slightly firmer, carefully transfer them to a wire cooling rack to cool completely.