Preheat Oven: Set your oven to 160ºC (320ºF) with a fan or 170ºC (338ºF) for conventional heating. Line a baking tray with parchment paper.

Melt Chocolate and Butter: In a heatproof bowl, combine the chopped dark chocolate and butter. Place the bowl over a pan of simmering water, ensuring the bottom doesn't touch the water. Stir occasionally until melted and smooth.

Whip Eggs and Sugars: In a separate bowl, use an electric mixer to beat the caster sugar, light brown sugar, and eggs until the mixture is light, thick, and fluffy.

Combine Mixtures: Gently fold the melted chocolate and butter mixture into the whipped eggs and sugars until well combined.

Add Dry Ingredients: Sift in the cocoa powder, flour, baking powder, and salt. Fold the dry ingredients into the wet mixture until a smooth dough forms.

Portion Dough: Using a spoon or cookie scoop, place portions of dough onto the prepared baking tray, spacing them about 2 inches apart. Optionally, sprinkle a small amount of sea salt on top of each.

Bake: Bake in the preheated oven for 12 minutes, or until the edges are set and the centers are still slightly soft.

Cool: Allow the biscuits to cool on the baking tray for a few minutes before transferring them to a wire rack to cool completely.​