Melt 150g of the chocolate in short bursts in the microwave in a large jug
thaw the aqua faba if using.
whisk until stiff and around 400ml in volume
Add the rest of the wet ingredients to the same bowl or jug, mix well and heat a bit more in the microwave to keep the chocolate liquid. Don't overdo it though - short bursts on medium power.
Mix (with an electric whisk preferably) so it is completely homogenous.
Add all wet ingredients to the dry ingredients along with the aqua faba and mix/whisk well with an electric whisk.
Fold in (or use the electric whisk on medium) 50g of chocolate cut into small pieces (and cocoa nibs if using). Keep back 25g to scatter on top.
Pour mixture into a grease proof paper lined smaller rectangular and deep Pyrex or metal dish/tray (26cm by 16cm)
scatter the other 25g of chocolate pieces or mini buttons on top.
Cook at 160°C for 45 to 50 minutes
Check it’s done with a skewer – it should come out with no batter – but just a moist crumb. It will carry on cooking a little after you take it out of the oven anyway so not absolutely critical.
Lift out of the tray with the edges of the greaseproof paper and cool on a wire rack.
Cut when cooled with a sharp knife.