Place the flour, Guittard Cocoa Powder, baking soda and salt into a bowl and mix together to combine. Separately, add the butter and sugars into a large bowl and using a mixer to beat together for a couple minutes until everything is combined.
Add the flour mixture and on low speed mix until the flour has just been incorporated into the butter, stopping when the dough is still very crumbly but only once you can no longer see any flour.
Tip the crumbly dough out on the work surface and carefully bring together into a dough. Divide into two equal pieces and lightly form into logs, about 5cm thick.
Wrap the dough in clingfilm and refrigerate for a few hours until firm. Preheat the oven to 180˚C (160˚C fan) and line two baking trays with parchment paper.