Measure out your butter and allow to soften to room temperature before starting.
In a large bowl, cream together softened butter with the white sugar and salt, until light and fluffy.
Beat in eggs until fully incorporated.
In a separate bowl, sieve together the flour, cocoa powder, and baking soda. Blend well.
Add the dry ingredients to the wet ingredients, and mix together until combined.
Turn the dough out onto your surface and push together into a flat square.
Wrap the dough in plastic wrap and refrigerate for 1 hour.
Preheat oven to 160˚C (325˚F).
Remove the dough from the fridge, and for ease of rolling out, divide the dough into 2 (this way the dough stays cool longer and prevents the need of re-chilling later on).
To roll out the dough, place half of the dough between two sheets of parchment paper (alternatively the dough can be rolled out on a lightly floured surface - though the parchment is preferred as it prevents any additional flour from altering the color/appearance of the dark cookies).
Roll the dough between the two sheets of parchment to ½ cm (¼-inch) thickness.
Cut the dough in whatever shape you want. If you're making a cake, use the mold to get the approximate size. Do not that dough will expand a little as it cooks.
Bake in a preheated oven for 15minutes.
Remove and transfer cookies to a cooling rack to cool completely.
If allowed to sit in the open air for a few hours, it will harden like a cookie.