Preheat the oven to 180℃/160℃ fan/350℉/gas mark 6. Line a 20 cm/8 in square cake tin with baking paper.
Stir the flour, cocoa powder and sugar together in a bowl, make sure it is well combined and free from lumps.
250 g (2 cups) plain (all-purpose) flour, 60 g (⅔ cup) cocoa powder, 175 g (¾ cup + 2 Tablespoons) caster or granulated sugar
Add the melted butter and stir until well combined, it will be quite a dry mixture but make sure that all of the dry ingredients are moistened.
135 g (10 Tablespoons) vegan block butter
Tip the mixture into the prepared tin and spread it out in an even layer. Use your hands, the back of a spoon or the base of a glass to pack the crumbs down very firmly and evenly. Make sure that you really press it down firmly otherwise the concrete will be overly crumbly.
Lightly brush or sprinkle the surface with a little water then bake the concrete for about 25 minutes* until dry to the touch. It will still be a little soft but will firm up as it cools.
As soon as you take it out of the oven, sprinkle generously with sugar. Allow to cool in the tin for 5-10 minutes, then use a sharp knife to cut it into triangles or squares while it is still warm. Allow to cool on a wire rack before serving.