Preheat the oven to 350°F / 180°C. Line a half sheet pan with parchment paper and trace 8 circles on it, turning the parchment over so the pencil side is facing down.

Fit a piping bag with a medium round piping tip or use a ziploc bag with the corner cut off. Place clips in the piping bag to prevent leaking.

In a heatproof bowl over a pan of simmering water, combine the dark chocolate and butter. Heat until melted and smooth, or microwave in 30-second increments, stirring after each.

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

In a large bowl, combine the egg, granulated sugar, and brown sugar. Mix on medium to high speed for 4 minutes until lightened 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.

Pipe the mixture onto the parchment paper, holding the tip about 2 cm above. Pipe a dome the diameter of the traced circles.

Bake the cookies for 10 minutes until just set and crinkly on top. Do not overbake.

Let cool completely on the pan before transferring to a storage container or serving.