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 and milk/water and set aside to thicken to make a chia/flax egg.
In a separate small bowl, whisk together the milk and the apple cider vinegar. Set aside to thicken/activate & turn into 'buttermilk'.
In a large bowl, whisk together all of the dry ingredients except the nuts and chocolate, then toss the chopped chocolate and pecans through if using. Set aside.
Place the chocolate and vegan butter in a large heat‐proof mixing bowl and 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 and 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 and whisk well to begin dissolving the sugar. Whisk in the vanilla paste or extract followed by the chia egg and finally the buttermilk. Whisk together until well combined.
Pour the wet mixture over the dry mixture and stir until combined. Scoop the batter into the prepared pan and spread with your hands or the back of a spoon until smooth and even.
Bake for about 40-55 minutes (I give the pan a shake occasionally to make sure that crackly crust happens!) until the middle is less wobbly, and a skewer inserted into the middle comes out with a bit of gooey batter attached (but not too much). TIP: Bake for the lesser amount if you like insanely gooey brownies and the skewer should have more raw batter attached.
Remove from the oven and set the pan of brownies on a wire rack – leave in the pan until completely cool. Once cooled, carefully remove the brownies from the pan using the overhanging baking paper.
Dust with a little cocoa powder or icing sugar and sprinkle with flaky salt, then slice into 16 squares with a sharp, hot knife. Serve warm or cold.