Cream together the softened butter and sugar until light and fluffy.
Beat in eggs until smooth.
In a separate bowl, mix the flour, cocoa and baking soda together and add to the wet ingredients.
Mix until a dough forms.
Flatten the dough into a flat circle or square, wrap in plastic wrap and refrigerate for an hour.
Preheat the oven to 160°C or 325°F and line two baking trays with baking paper and set aside. Remove the dough from the fridge and divide the dough into 4 (this way the dough stays cool longer and prevents the need of re-chilling later on).
Place a quarter of the dough on a lightly floured bench and roll out the dough to around about a 1/4 inch thickness. (Alternatively you can roll out the dough between two sheets of parchment paper).
Using a small round cookie cutter (Tasty suggest the rim of a champagne glass is the perfect size) cut the dough into rounds and place on the prepared baking trays, leaving at least 1/2 an inch between each cookie.
Re-roll out any scraps to cut additional cookies. Repeat with this process with each remaining pieces of dough.
Bake for 15-20 minutes. (I baked for 15 but would suggest baking for 20 if you want a crisp Oreo-like cookie).
Allow the cookies to cool completely while you make the cream filling
To make the filling, beat the butter until smooth.
Add in the icing sugar and vanilla extract and beat until all combined and fluffy.
Spread a generous scoop of the icing onto one of the cookies and sandwich it with another. Give it a very light press and scrape any excess off to clear and even out the sides.