Preheat the oven to 350°F (175°C). In a medium saucepan over low heat, melt the dark chocolate and butter together for 3-5 minutes.

In a separate bowl, whisk together the eggs, demerara sugar, and caster sugar until pale and fluffy, about 5 minutes.

Sift together cocoa powder and plain flour in another bowl.

Slowly incorporate the melted chocolate-butter mixture into the egg-sugar mixture while stirring gently.

Fold in the sifted flour and cocoa mixture until just combined; add chocolate chips if using.

Drop rounded mounds of dough onto a parchment-lined baking sheet and bake for 8-10 minutes until edges are set.

Allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.