Preheat the oven to 180° C / 355° F. Line a square 20 cm / 8 inch tin with baking paper.

Place ground flax in a large mixing bowl with 120 ml / ½ cup of plant milk and set aside for 10 minutes.

Prepare tahini whip and melt the chocolate (see next step) while waiting. Pop tahini into a medium bowl and gradually add in water while whisking vigorously - add only as much as needed (my tahini was quite stiff so I needed 4 tbsp) - you want a thick, smooth mixture that barely falls off the spoon.

Melt broken up chocolate and vegan butter in a bowl suspended over a pot of barely simmering water. Allow them to melt slowly, switch the heat off when most of the chocolate has melted. Stir gently to combine at the end.

Add both sugars to the activated flax and whisk with an electric whisk for 2 minutes - until the mixture becomes bubbly and thick - see photo above.

Gently fold warm (not cooled) chocolate mixture into the flax-sugar mixture.

Place a sieve over the bowl and sift in dry ingredients in two batches whilst gently folding with a spatula. Keep on folding until there is no dry pockets left but no more than that.

Transfer the batter into the baking tin, spread to the edges with a spatula.

Dot the top with tahini mixture and, using a chopstick, create decorative swirls. Sprinkle with sesame seeds if using.

Bake for 22 minutes - the brownies should have a bit of a wobble when being taken out of the oven.

Cool completely and then place in the fridge for a few hours for the brownies to firm up and become fudgy.