Place the aquafaba and cream of tartar (if using) in the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl, if using a handheld electric whisk). Whisk on high speed until it becomes a thick pale foam, like whipped egg whites.

Whilst continuing to whisk, add the sugar a few tablespoons at a time.

Once all the sugar has been added, continue to whisk for 5 minutes more to ensure as much sugar as possible has dissolved. The mixture will still feel slightly grainy if you rub some of it between your fingertips (if it feels VERY grainy, keep whisking to allow more of the sugar to dissolve). It should look glossy, thick and opaque white, like egg whites whisked to semi-stiff peaks.

NB: I've noticed that sometimes the aquafaba doesn't get AS thick as egg whites after whisking, i.e. it doesn't hold a peak when the whisk is lifted out. But as long as the mixture is opaque and roughly tripled in volume when you're done whisking, it should be fine to use.

Place the vegan butter and broken up chocolate into a small pot and place over a low heat. Stir until almost fully melted. Remove from the heat and set aside so the residual heat can melt it all fully.

Once fully melted, stir the sourdough discard and vanilla extract into the pot of melted chocolate/butter mixture. It may look kind of split/grainy but this is fine.

Preheat the oven to 180°C (350°F) fan or 200°C (400°F) non-fan. Line a 7.5 x 9.75-inch (19 x 25 cm) rectangular OR an 8 or 9-inch (20 or 23cm) square brownie pan with baking paper.

Mix around ¼ of the whisked aquafaba into the melted chocolate mixture. You don't have to be gentle here as this step is to help loosen the texture of the chocolatey mixture.

Now pour that loosened chocolatey mixture into the bowl of whisked aquafaba. Sift the cocoa powder, ground almonds, cornflour, bicarb and salt on top (see notes if you don't have a sieve).

Use a silicone spatula to fold the mixture together gently, trying to maintain as much of that air in there as possible. Make sure you get right to the bottom of the bowl and scrape the sides too!

Pour the batter into the prepared pan and bake for 25-30 minutes - the top should look dry, matte and the brownies shouldn't wobble when you shake the pan. If you insert a toothpick into the centre, it should come out with some thick, gooey batter (NOT loose, drippy batter!) attached to it.

Remove from the oven and run a knife around the edges whilst they're hot to loosen any bits which may be stuck. Leave to cool for 20-30 minutes before removing from the tray and cutting into squares. They will sink in the centre as they cool so may crack a bit as this happens. You can sprinkle them with some flaky salt as well now.