Heat your oven to 180°C (350°F/gas mark 4), and line an 18cm x 28cm (7” x 11”) oven dish or brownie pan with parchment paper.
Over a low heat, melt 150g chocolate and the vegan margarine in a medium-sized saucepan.
Once melted, whisk in the plant milk, until it’s fully incorporated.
Mix together the flour, cocoa, sugar, ground flax seed, baking soda, and sea salt in a large bowl.
Add the chocolate mixture, water, and aquafaba to the dry ingredients, and mix well, until everything is incorporated, and you have a stiff batter. Add the rest of the chocolate. (I use a stand mixer.)
Tip the batter into your prepared oven dish.
Smooth the top with a wet spatula, so that it’s even (or use the back of a large spoon).
Bake in the centre of the oven for 30 mins. The top will be firm but the centre will still be a bit gooey – this is intentional!
Leave the brownies to cool in the tray, then cut into 18 pieces.
Store in an airtight container at room temperature for up to a week.
If you don’t have caster sugar (sometimes called bar sugar or superfine granulated), you can make your own by whizzing normal granulated sugar in a food processor. The aim is have it somewhere between granulated and powdered.
Or use ½ teaspoon baking powder + ¼ teaspoon bicarbonate of soda instead of baking soda.
Re. thick aquafaba (AF): I find that butter beans yield the best AF, in terms of viscosity; however, if you only have thin AF, you can make it thicker it by rapidly boiling, in order to reduce it down.