Preheat the oven to 180 °C/ 350 °F. Line a large baking tray or cookie sheet with greaseproof paper.
Prepare the flaxseed egg in a small bowl by mixing 1 tbsp ground flaxseed with 2.5 tbsp water. Set aside to thicken.
In a medium-sized mixing bowl, cream together the vegan butter/margarine, coconut sugar and white sugar until soft and fluffy.
Add the flaxseed egg and vanilla extract to the vegan butter-sugar mixture and mix to combine. If at this stage the mixture looks a little curdled, don't worry!
Add the plain flour, cocoa powder and baking soda to the bowl. Fold in with a spatula until combined. The mixture should be quite wet, like a thick cake batter consistency.
Add heaped tablespoons of the cookie mixture to the lined baking tray. Place the cookies quite far apart (no more than 6 on one tray) as they will spread in the oven. Also try to keep your cookie balls as round as possible - if you have an ice cream scoop it is great for this task.
Bake the cookies for 10 minutes until slightly golden at the edges. I'd check the cookies at the 8 minute mark as ovens vary.
Leave the cookies on the tray for a few minutes before transferring to a wire rack to cool completely.