Put the butter in the bowl of an electric mixer with a paddle attachment fitted and beat on a medium speed until the butter is creamy.
Add the brown and white sugars, salt and vanilla extract and beat for a couple of minutes.
Reduce the mixer to a low speed and sift in the flour, cocoa and baking soda and beat until combined. Note the mixture will look crumbly, but this is normal.
Turn the mixture our onto some cling film and squeeze the dough into a ball and then divide into tow. Form each ball into a log and wrap up in cling film and put in the fridge to chill for at least 2 hours.
Preheat the oven for 165 degrees celsius or 325 farenheit and move an oven rack to the center of the oven.
With a sharp knife slice the logs into rounds about 1.5cm or ½ an inch thick. If the cookies break when sliced just squash them back together again. Place each cookie onto a non-stick baking tray or baking tray lined with baking paper.
Bake each cookie for 12 minutes. Note that the cookies will not look cooked, but they will firm when they cool.
The cookies can be kept for up to 3 days at room temperature or frozen for up to a month. The dough can be frozen, no need to defrost before baking, however add 1 minute to the baking time.