
Preheat oven to 180 C (350 F) standard / 160 C (320 F) fan-forced. Line two baking trays with baking or parchment paper.
In a large mixing bowl, add cold butter and sugar. Beat with an electric mixer until combined and creamy. Add egg. Beat again until combined – but try not to over mix.
Add flour, cocoa, baking soda and salt and beat on a low speed until soft cookie dough forms.
Add all the chocolate and beat briefly. Scoop out cookie dough, roughly 2 tablespoons per cookie, and place on prepared oven trays, leaving room for the cookie to spread.
Bake cookies for 12-14 minutes. Leave cookies to cool completely. They will firm up as they cool.