Step 1

Preheat the oven to 180° C / 355° F (160° C / 320° F with a fan setting). Grease the sides of a 20 cm / 8 inch baking tin and line the bottom - I used an adjustable baking frame on a sturdy baking tray.

Step 2

Place broken up chocolate and chopped vegan butter in a metal or glass bowl suspended over a water bath. Make sure the bowl does not touch the water underneath and use low heat. Once the water under the bowl starts to simmer, switch off the heat and allow the contents of the bowl to melt gently into each other in the residual heat. GENTLY and slowly mix them together.

Step 3

Whip aquafaba with an electric whisk until you get a dense foam, full of little bubbles (there is no need to achieve stiff peaks for this recipe).

Step 4

Gradually add sugar to the aquafaba, whisking well after each addition.

Step 5

Slowly pour very warm (otherwise the batter will become too stiff to add all of the flour) chocolate and vegan butter mixture into the aquafaba mixture and gently fold in until uniform.

Step 6

Mix flour, salt, cocoa and baking soda in a small bowl. Fold them in two batches, until no dry pockets remain.

Step 7

Transfer brownie batter into the prepared baking tin, spread it into the corners with a spatula. Bake for 17-18 minutes, allow to go cold before topping with the mousse.

Step 8

Once the base is cold, gently remove it from the tin. Line the sides the the tin with baking paper and put the brownie base back in.

Step 9

Place chocolate and vegan butter in a metal or glass bowl suspended over a water bath. Make sure the bowl does not touch the water underneath and use low heat. Go to the next step while you wait.

Step 10

Place aquafaba and cream of tartar in a spotlessly clean bowl.

Step 11

Whisk on medium/high speed until you reach stiff peaks (when you invert the bowl, whipped aquafaba should stay put). Depending on your equipment, it may take around 8-10 minutes (it does for me and I have an old handheld whisk).

Step 12

Add sugar in 1 tbsp increments while whipping and slowly counting to 20 after each addition.

Step 13

Once the chocolate and butter have melted, stir them GENTLY into each other.

Step 14

Add a large spoonful of whipped aquafaba to the melted chocolate and fold it in very gently.

Step 15

Now, add about a third of melted chocolate into the whipped aquafaba and fold gently using a flexible spatula, when it's roughly incorporated add another third and then the last third.

Step 16

When you've added all of the chocolate, carry on folding gently until no white streaks remain. Be gentle and slow or else you'll knock too much air out of the aquafaba.

Step 17

Top cold brownie with the mousse and place in the fridge until set - at least overnight. Decorate* with cocoa powder and cut with a sharp knife.