Preheat the oven to 180C or 356F. Put a tiny dab of oil in the 4 corners of 2 baking sheets and line them with baking paper. The oil will stop the baking paper moving around when you spoon the cookie batter on.
In a small saucepan, combine the water and sugars and stir. Place over a low-medium heat and gently cook until the sugar has completely dissolved, around 3 minutes. Take it off the minute it’s ready, don’t let it bubble and start caramelising.
In a small heatproof bowl, combine the chocolate and vegan butter. Place it atop another small saucepan half filled with water, ensuring the water doesn’t touch the bottom of the bowl. Place over a medium heat and cook, stirring intermittently, until the chocolate has melted. Remove from the heat.
In a small bowl, mix together the cocoa, buckwheat powder, baking powder/soda and salt. This will give the chocolate and sugar syrup a few minutes to cool. Then, whisk in the flour mixture until totally smooth, around 1 minute. Add the vanilla bean paste and whisk again to combine.
Use a spoon to make 10-12 circular cookies across the two trays. You can smooth over the tops a little if necessary. Place into the oven for 13-15 minutes. Allow to cool on the tray before gently removing, and allow to cool before placing in an airtight container. These cookies store really well for at least a week.