*If you only have one 20 cm/8 in tin then you will need to make a HALF BATCH of the cookie batter, bake that and then make another half batch. Half batch quantities can be found below in the notes section of the recipe card.*
Preheat the oven to 180℃/160℃ fan/350℉/gas mark 4. Grease two 20 cm/8 in square cake tins with vegan butter and line them with baking paper. (Sticking the parchment to the tin with butter makes it easier to spread the batter later).
Whisk together the melted butter, sugar and cocoa powder with a balloon whisk until smooth.
70 g (5 Tablespoons) vegan block butter, 120 g (½ cup + 2 Tablespoons) caster or granulated sugar, 20 g (¼ cup) black cocoa powder or Dutch processed cocoa powder
Whisk in the milk, vanilla, salt and baking powder, followed by the plain flour.
80 g (⅓ cup) unsweetened non-dairy milk (I use soy), ½ teaspoon vanilla extract, ¼ teaspoon salt, ½ teaspoon baking powder, 130 g (1 cup + 1 Tablespoon) plain (all-purpose) flour
Divide the batter evenly between the tins (I use a scale for accuracy), then use an offset palette knife to spread it into a thin, even layer. It doesn't have to be perfect but it will not spread much in the oven so get it as even as you can.
Bake the cookies for 15 minutes until set but still slightly soft to the touch. Allow to cool in the tin for 10 minutes then lift them out using the baking paper and place on a wire rack to cool (no need to remove the paper).