Cut the vegan butter into small chunks and place it in a large bowl. Use an electric whisk or stand mixer to whisk the butter for 2 minutes. It will become light and airy. Add the demerara sugar, superfine sugar, and vanilla extract and whisk for another 2 minutes to cream everything together.
Add the flour, cocoa powder, arrowroot powder, baking soda, and salt to the bowl and whisk on low speed until the mixture comes together to form a clumpy dough consistency.
Knead the dough into a uniform ball, then flatten it down into a disc shape and wrap it with cling film. Place in the fridge to chill for at least half an hour (you can leave it overnight if preferred).
Preheat your oven to 160°C (320°F). Lightly dust your work surface and rolling pin with cocoa powder and carefully roll out the dough to approximately ¼ inch in height. Use a 2" cookie cutter to cut out cookies and place them on the lined baking tray, leaving about an inch between each cookie.
Bake for 14-15 minutes, then remove from the oven and allow the cookies to sit on the baking tray to cool for about 5 minutes. Carefully slide the sheet of parchment paper with the cookies off of the tray and onto a cooling rack. Cool for another 15-20 minutes until they reach room temperature before filling them.