Preheat oven to 160C/320F. Line a 22 cm/9 inch square pan with baking paper on all sides, leaving some overhang to use as a handle after baking (to lift the brownies out of the tin).
In a small bowl, whisk together the ground chia/flax & almond milk/water & set aside to thicken to make a chia/flax egg. In a separate small bowl, whisk together the almond milk & the apple cider vinegar. Set aside to thicken/activate & turn into 'buttermilk'. In a large bowl, whisk together the dry ingredients. Set aside.
Place the chocolate & vegan butter in a large heat‐proof mixing bowl & place over a saucepan filled with a few cm of simmering water. Allow to melt, stirring often so that the chocolate does not burn. Once melted & combined, remove from the heat (it only needs to just melt, don't leave it any longer or it will burn). Add the sugar straight away while the mix is still warm & whisk well to begin dissolving the sugar. Whisk in the vanilla extract followed by the chia egg & finally the buttermilk. Whisk together until well combined.
Pour the wet mixture over the dry mixture & stir until combined. Scoop the batter into the prepared pan & spread with your hands or the back of a spoon until smooth and even.
Bake for about 25-35 minutes (in the last 10 minutes I give the pan a shake every few minutes to make sure that crackly crust happens!) until crackly on top & a skewer inserted into the middle comes out a bit gooey with batter attached. Remove from the oven & set the pan of brownies on a wire rack – leave in the pan until completely cool (go & make the frosting as they bake & cool!). Once cooled, carefully remove the brownies from the pan using the overhanging baking paper.