Preheat the oven to 350?F (180?C) and line a half sheet pan with parchment paper.

Trace 8 circles on the parchment paper and turn it over.

Fit a piping bag with a medium round piping tip and secure to prevent batter leakage.

Melt the dark chocolate and butter in a heatproof bowl over simmering water or in the microwave until smooth.

Sift the cocoa powder over the melted chocolate and butter mixture and stir until combined. Let stand for 8 minutes.

While waiting, combine the egg, caster sugar, and brown sugar in a large bowl and mix on medium-high speed for 4 minutes until light and increased in volume.

Add the chocolate mixture to the egg and sugar mixture and mix to combine.

Add the flour and salt, incorporating by hand until smooth and lump-free.

Transfer the mixture to the piping bag and pipe onto the prepared parchment paper.

Bake the cookies for 10 minutes until just set and crinkly on top. Allow to cool completely on the pan before transferring to a storage container.